Support "simple" Closure compilation mode

[ochafik]

The Closure Compiler can almost consume the output of DDC with --language_in=ECMASCRIPT6_STRICT -O SIMPLE (and output ES5 code).

Here are some blockers:

• (fixed by google/closure-compiler@3f54e47) super getter calls aren't supported yet (Implement ES6 transpilation of super.getter calls google/closure-compiler#1089).

• (fixed) super method calls make the compiler to crash (Internal error with super method call google/closure-compiler#1125).

• super method calls on computed properties crash the compiler (Super calls on computed properties crash the compiler google/closure-compiler#1184)

• Cannot (re)define local Object, Error classes in core.js. This can be worked around by attaching the classes to some namespace (see branch closure-qualify):

  core.Object = class Object { ... };
  core.Deprecated = class Deprecated extends core.Object { ... };
  

• (fixed) let often makes the compiler to crash (Internal error with let inside while loop google/closure-compiler#1124). Could be somehow worked around by using var everywhere, but then the hand-written parts of the runtime are an issue.

• (fixed by e693a00) "The class in an extends clause must be a qualified name" (see workaround in branch closure-annotations-alias-parent).

• Cycles in ES6 module imports crash Closure. This can be worked around by using rollup.js to unfold the imports (fixed one blocker in 0.25.4), then running Closure on the result.

Other annoying non-blockers:

• (fixed) Misplaced annotations warning with computed properties (Misplaced annotation warning with computed properties in ES6 classes google/closure-compiler#1126)

(Note: "ADVANCED" optimizations are filed under #311)

[jmesserly]

thanks for collecting all of these!

Cannot (re)define local Object, Error classes in core.js. This can be worked around by attaching the classes to some namespace (here core is the current exports var):

Nice! Yeah that would work & also would simplify our JsSymbol/Symbol issues.

let often makes the compiler to crash (google/closure-compiler#1124). Could be somehow worked around by using var everywhere, but then the hand-written parts of the runtime are an issue.

note: let is needed for correct behavior (e.g. closures in loops), so we can't compile to var as a workaround. Hopefully they can get that crashing bug fixed...