Fix problems with async* and await for in dart2js.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 31c2fddbf19cdf8c328fd5dc3de0e7dd5779d34d..70fb138c5a71b299923f01c373bcc77dcf0299b2 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -6033,6 +6033,7 @@ class SsaBuilder extends ResolvedVisitor {
     // body-block will be used, but for loops we will add (unnecessary) phis
     // that will reference the body variables. This makes it look as if the
     // variables were used in a non-dominated block.
+    LocalsHandler savedLocals = new LocalsHandler.from(localsHandler);
     HBasicBlock enterBlock = openNewBlock();
     HTry tryInstruction = new HTry();
     close(tryInstruction);
@@ -6055,6 +6056,7 @@ class SsaBuilder extends ResolvedVisitor {
 
     SubGraph finallyGraph = null;
 
+    localsHandler = new LocalsHandler.from(savedLocals);
     startFinallyBlock = graph.addNewBlock();
     open(startFinallyBlock);
     buildFinally();
@@ -6098,6 +6100,7 @@ class SsaBuilder extends ResolvedVisitor {
     // block and the finally block.
     addExitTrySuccessor(startFinallyBlock);
 
+    localsHandler = savedLocals;

[floitsch, 2015/03/16 13:33:50]

I'm not 100% sure that one is correct (even in the original code).
It probably doesn't matter, because we box all try-variables, but it feels
wrong.

Copy over the comment from the original visitTry.

Maybe add a TODO: "we can probably do this, because try-variables are boxed.
Need to verify". or something similar.

[sigurdm, 2015/03/16 14:08:09]

Done.

     open(exitBlock);
     enterBlock.setBlockFlow(
         new HTryBlockInformation(