Fix compilation with Dart 1.8.0. (left some stubs as TODO)
Use C++11 unique_ptr instead of adhoc scoped_ptr
Implement file read/write callbacks with std streams.
Introduce AutoEnterIsolate and AutoEnterScope to simplify scoped code.
Merge branch 'update-dart-1.8.0' into ccompile

# Conflicts:
#	lib/src/dart.cc
#	lib/src/isolate.cc
#	lib/src/scoped_ptr.h