I just did a clean clone of the repository, and here is the output:

Stack: Error
    at assertTrue (eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8))
    at eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8)
    at walkAndModifyInterdata (eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8))
    at walkAndModifyInterdata (eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8))
    at walkAndModifyInterdata (eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8))
    at walkAndModifyInterdata (eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8))
    at eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8)
    at Array.forEach (native)
    at eval at globalEval (/home/joshua/Development/Emscripten/emscripten/src/compiler.js:105:8)
    at Array.forEach (native)

undefined:60
    throw msg;
          ^
Assertion failed: We can only unfold illegal constants in phis
Traceback (most recent call last):
  File "/home/joshua/Development/Emscripten/emscripten/emscripten.py", line 800, in <module>
    _main(environ=os.environ)
  File "/home/joshua/Development/Emscripten/emscripten/emscripten.py", line 788, in _main
    temp_files.run_and_clean(lambda: main(
  File "/home/joshua/Development/Emscripten/emscripten/tools/tempfiles.py", line 38, in run_and_clean
    return func()
  File "/home/joshua/Development/Emscripten/emscripten/emscripten.py", line 796, in <lambda>
    DEBUG_CACHE=DEBUG_CACHE,
  File "/home/joshua/Development/Emscripten/emscripten/emscripten.py", line 683, in main
    jcache=jcache, temp_files=temp_files, DEBUG=DEBUG, DEBUG_CACHE=DEBUG_CACHE)
  File "/home/joshua/Development/Emscripten/emscripten/emscripten.py", line 266, in emscript
    assert len(output) == 2, 'Did not receive forwarded data in an output - process failed? We only got: ' + output[0][-3000:]
AssertionError: Did not receive forwarded data in an output - process failed? We only got:       HEAP32[(($341)>>2)]=$339;
      var $342=HEAP32[(($331)>>2)];
      var $343=((($342)-(1))|0);
      var $344=(($264+36)|0);
      var $345=$344;
      HEAP32[(($345)>>2)]=$343;
      var $346=((($339)-(1))|0);
      var $347=(($264+40)|0);
      var $348=$347;
      HEAP32[(($348)>>2)]=$346;
      var $349=HEAP32[(($274)>>2)];
      var $350=(($349+12)|0);
      var $351=HEAP32[(($350)>>2)];
      var $352=$351;
      var $353=HEAP32[(($352)>>2)];
      var $354=(($353+32)|0);
      var $355=HEAP32[(($354)>>2)];
      var $356=FUNCTION_TABLE[$355]($351);
      var $357=(($264+4)|0);
      var $358=$357;
      HEAP32[(($358)>>2)]=$356;
      var $359=HEAP32[(($274)>>2)];
      var $360=(($359+12)|0);
      var $361=HEAP32[(($360)>>2)];
      var $362=$361;
      var $363=HEAP32[(($362)>>2)];
      var $364=(($363+36)|0);
      var $365=HEAP32[(($364)>>2)];
      var $366=FUNCTION_TABLE[$365]($361);
      var $367=(($264+8)|0);
      var $368=$367;
      HEAP32[(($368)>>2)]=$366;
      var $369=(($264+44)|0);
      HEAP8[($369)]=0;
      var $370=(($264+45)|0);
      HEAP32[(($265)>>2)]=(((5438052)|0));
      HEAP8[($370)]=1;
      var $_0_in = $264;
      var $_0_in;
      var $_0=$_0_in;

      return $_0;
    } else {
      var $282=FUNCTION_TABLE[$279]($275);
      var $283=(($264+28)|0);
      var $284=$283;
      HEAP32[(($284)>>2)]=$282;
      var $285=HEAP32[(($274)>>2)];
      var $286=(($285+12)|0);
      var $287=HEAP32[(($286)>>2)];
      var $288=$287;
      var $289=HEAP32[(($288)>>2)];
      var $290=(($289+12)|0);
      var $291=HEAP32[(($290)>>2)];
      var $292=FUNCTION_TABLE[$291]($287);
      var $293=(($264+32)|0);
      var $294=$293;
      HEAP32[(($294)>>2)]=$292;
      var $295=HEAP32[(($284)>>2)];
      var $296=((($295)-(1))|0);
      var $297=(($264+36)|0);
      var $298=$297;
      HEAP32[(($298)>>2)]=$296;
      var $299=((($292)-(1))|0);
      var $300=(($264+40)|0);
      var $301=$300;
      HEAP32[(($301)>>2)]=$299;
      var $302=HEAP32[(($274)>>2)];
      var $303=(($302+12)|0);
      var $304=HEAP32[(($303)>>2)];
      var $305=$304;
      var $306=HEAP32[(($305)>>2)];
      var $307=(($306+32)|0);
      var $308=HEAP32[(($307)>>2)];
      var $309=FUNCTION_TABLE[$308]($304);
      var $310=(($264+4)|0);
      var $311=$310;
      HEAP32[(($311)>>2)]=$309;
      var $312=HEAP32[(($274)>>2)];
      var $313=(($312+12)|0);
      var $314=HEAP32[(($313)>>2)];
      var $315=$314;
      var $316=HEAP32[(($315)>>2)];
      var $317=(($316+36)|0);
      var $318=HEAP32[(($317)>>2)];
      var $319=FUNCTION_TABLE[$318]($314);
      var $320=(($264+8)|0);
      var $321=$320;
      HEAP32[(($321)>>2)]=$319;
      var $322=(($264+44)|0);
      HEAP8[($322)]=0;
      var $323=(($264+45)|0);
      HEAP32[(($265)>>2)]=(((5437996)|0));
      HEAP8[($323)]=1;
      var $_0_in = $264;
      var $_0_in;
      var $_0=$_0_in;

      return $_0;
    }
  }
}

Assertion failed: We can only unfold illegal constants in phis

Traceback (most recent call last):
  File "/home/joshua/Development/Emscripten/emscripten/emcc", line 1435, in <module>
    final = shared.Building.emscripten(final, append_ext=False, extra_args=extra_args)
  File "/home/joshua/Development/Emscripten/emscripten/tools/shared.py", line 985, in emscripten
    assert os.path.exists(filename + '.o.js') and len(open(filename + '.o.js', 'r').read()) > 0, 'Emscripten failed to generate .js: ' + str(compiler_output)
AssertionError: Emscripten failed to generate .js: