[NEW TYPE INFERENCE] Turn on default externs for unit tests.

Also fix associated failures:
* @struct/@dicts should not warn for subclassing Object
(even though it isn't @struct/@dict)
* Don't give NON_NUMERIC_ARRAY_INDEX warning for loose objects
(they are subclasses of Array, but we shouldn't treat them
as real Arrays)
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=74157370
Fixes in the parsing of for/in and for/of statements:
- Allow destructuring patterns, e.g. for ([x, y] of pairs) {}
- Disallow assignement, e.g. for (a=1 in b) {}
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=74158340
Don't crash on a something.base() call with no corresponding goog.inherits.

Fixes #596
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=74161726