Implementation of async-await transformation on js ast.

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

Index: pkg/compiler/lib/src/ssa/ssa_tracer.dart
diff --git a/pkg/compiler/lib/src/ssa/ssa_tracer.dart b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
index 09d8069766eaba9cb227e1f1ae82953d0d1c6031..103ecae03cd55dbda7f75be8f913491d8038dd72 100644
--- a/pkg/compiler/lib/src/ssa/ssa_tracer.dart
+++ b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
@@ -519,4 +519,12 @@ class HInstructionStringifier implements HVisitor<String> {
   String visitDynamicType(HDynamicType node) {
     return "DynamicType";
   }
+
+  String visitAwait(HAwait node) {
+    return "await ${temporaryId(node.inputs[0])}";
+  }
+
+  String visitYield(HYield node) {
+    return "yield${node.hasStar ? "*" : ""} ${temporaryId(node.inputs[0])}";
+  }
 }