Rewire DDC to use the analyzer task model

lib/runtime/dart/_foreign_helper.js

Index: lib/runtime/dart/_foreign_helper.js
diff --git a/lib/runtime/dart/_foreign_helper.js b/lib/runtime/dart/_foreign_helper.js
index 166c7e190744f5463d6862e98106bf8312821187..81da65fbf7be58022304de11123a833d13261a2a 100644
--- a/lib/runtime/dart/_foreign_helper.js
+++ b/lib/runtime/dart/_foreign_helper.js
@@ -118,7 +118,7 @@ dart_library.library('dart/_foreign_helper', null, /* Imports */[
     constructors: () => ({JS_CONST: [JS_CONST, [core.String]]})
   });
   function JS_STRING_CONCAT(a, b) {
-    return a + b;
+    return dart.as(a + b, core.String);

[Jennifer Messerly, 2015/09/19 00:18:46]

what happened here? This returns String:

String JS_STRING_CONCAT(String a, String b) {
  // This body is unused, only here for type analysis.
  return JS('String', '# + #', a, b);
}

Do we not implement JS builtin anymore? If not, can we at least just trust it?

[Jennifer Messerly, 2015/09/19 00:27:51]

Ah, could be we lost this code:
https://github.com/dart-lang/dev_compiler/blob/94d823b08e1a6420823bc7bc894fa4...

note, that code's not exactly right either, it only happens to work for
dart:core types. Ideally we'd lookup the "type string" in the context where it
occurs, likely when we're doing name resolution.

Either that or just trust JS annotations, and infer their type from downward
inference. We have to trust them anyway, since we aren't typechecking the JS
code :). So hmm, maybe my vote is actually just to trust and ignore that type
string.

[Leaf, 2015/09/21 17:15:59]

Yes, I have't pulled this over yet.  I was a little unsure about what to do
here, since the comments indicate that the code was pulled over from the
analyzer, so I wanted to avoid duplicating it if it was already present. 
Clearly it's not kicking in if it is present though.  Does anyone know the
history of that code and the comment?

[Jennifer Messerly, 2015/09/21 18:18:59]

yup, see here:
https://github.com/dart-lang/sdk/blob/f6e0a8f30b32af16f9fede4b65698ebfacb28d9...

note that it only uses propagatedType. We also tweaked our version: restricted
to only dart:_foreign_helper and it supports the dart2js syntax of "bottom".

Either way, the lookup is pretty bad, it only works for types in dart:core.

   }
   dart.fn(JS_STRING_CONCAT, core.String, [core.String, core.String]);
   // Exports: