dart2js: Use frequency of occurence to sort metadata indices.

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 e40a6b838ac1dea110fc78cdacc932f3dbfa99e5..ef858a620e98dbc37a6080edf4ae79d57579858e 100644
--- a/pkg/compiler/lib/src/constants/values.dart
+++ b/pkg/compiler/lib/src/constants/values.dart
@@ -603,20 +603,20 @@ class InterceptorConstantValue extends ConstantValue {
   }
 }
 
-// TODO(johnniwinther): Remove this class.
 class DummyConstantValue extends ConstantValue {

[Johnni Winther, 2015/05/28 12:07:17]

Rename to SyntheticConstantValue, ForeignConstantValue or BackendConstantValue,
or ...

[herhut, 2015/06/01 12:09:41]

I went for SyntheticConstantValue.

-  final ti.TypeMask typeMask;
+  final payload;

[sra1, 2015/05/27 19:38:50]

TL;DR: should this class be split into 2 to 3 classes?

The original dummy constant was only copied (e.g. passed as an argument to an
unused parameter).  It did not matter at all what the textual JS representation
was.

If there is a different kind of 'constant' that actually flows to a nontrivial
use I think it should be a different kind of ConstantValue.  When 'payload' is
an expression, what how is that expression used at runtime?

I see that some uses of payload are js.Expression and some are TypeMask.  Are
there other uses?

[herhut, 2015/06/01 12:09:41]

The idea is that these constants are opaque to the constant system and only the
specific system that uses them, e.g. the JavaScript backend, knows which exist
and what they are. 

Currently we have
  DUMMY_INTERCEPTOR
  EMPTY_VALUE
  TYPEVARIABLE_REFERENCE

Only the last is new, the other two existed before.

+  final String kind;
 
-  DummyConstantValue(this.typeMask);
+  DummyConstantValue(this.kind, this.payload);
 
   bool get isDummy => true;
 
   bool operator ==(other) {
     return other is DummyConstantValue
-        && typeMask == other.typeMask;
+        && payload == other.payload;
   }
 
-  get hashCode => typeMask.hashCode;
+  get hashCode => payload.hashCode;
 
   List<ConstantValue> getDependencies() => const <ConstantValue>[];
 
@@ -624,9 +624,9 @@ class DummyConstantValue extends ConstantValue {
 
   DartType getType(CoreTypes types) => const DynamicType();
 
-  String unparse() => 'dummy($typeMask)';
+  String unparse() => 'dummy($kind, $payload)';
 
-  String toStructuredString() => 'DummyConstant($typeMask)';
+  String toStructuredString() => 'DummyConstant($kind, $payload)';
 }
 
 class ConstructedConstantValue extends ObjectConstantValue {