dart2js cps: Clean up and avoid processing unreachable code.

pkg/compiler/lib/src/constants/values.dart

Index: pkg/compiler/lib/src/constants/values.dart
diff --git a/pkg/compiler/lib/src/constants/values.dart b/pkg/compiler/lib/src/constants/values.dart
index 24b6370024005d7d1c2dc8fe968f434a150321e3..2a7c5c4d45e157f3f8896c95aa8fbf6f8cf2aec0 100644
--- a/pkg/compiler/lib/src/constants/values.dart
+++ b/pkg/compiler/lib/src/constants/values.dart
@@ -375,6 +375,9 @@ class StringConstantValue extends PrimitiveConstantValue {
       : this.primitiveValue = value,
         this.hashCode = value.slowToString().hashCode;
 
+  StringConstantValue.fromString(String value)
+      : this(new DartString.literal(value));
+
   bool get isString => true;
 
   DartType getType(CoreTypes types) => types.stringType;
@@ -748,4 +751,4 @@ class NonConstantValue extends ConstantValue {
 
   @override
   String unparse() => '>>non-constant<<';
-}
+}