Flatten _Future at runtime as well

Fixes #551

BUG=
[email protected]

Review URL: https://codereview.chromium.org/1998843002 .

Author: leafpetersen

pkg/dev_compiler/lib/runtime/dart_sdk.js

@@ -15397,7 +15397,7 @@ dart_library.library('dart_sdk', null, /* Imports */[
   const _prependListeners = Symbol('_prependListeners');
   const _reverseListeners = Symbol('_reverseListeners');
   const _removeListeners = Symbol('_removeListeners');
-  async._Future$ = dart.generic(T => {
+  async._Future$ = dart.flattenFutures(dart.generic(T => {
     class _Future extends core.Object {
       new() {
         this[_zone] = async.Zone.current;
@@ -15886,7 +15886,7 @@ dart_library.library('dart_sdk', null, /* Imports */[
       names: ['_chainForeignFuture', '_chainCoreFuture', '_propagateToListeners']
     });
     return _Future;
-  });
+  }));
   async._Future = async._Future$();
   async._Future._INCOMPLETE = 0;
   async._Future._PENDING_COMPLETE = 1;

pkg/dev_compiler/lib/src/compiler/code_generator.dart

@@ -825,7 +825,8 @@ class CodeGenerator extends GeneralizingAstVisitor
     assert(formals.isNotEmpty);
     var genericCall = js.call('dart.generic((#) => { #; return #; })',
         [_emitTypeFormals(formals), body, element.name]);
-    if (element.type.isDartAsyncFuture) {
+    if (element.library.isDartAsync &&
+        (element.name == "Future" || element.name == "_Future")) {
       genericCall = js.call('dart.flattenFutures(#)', [genericCall]);
     }
     var genericDef = js.statement(