dart2js: Use frequency of occurence to sort metadata indices.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.constants.expressions;
 
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart2jslib.dart' show assertDebugMode, Compiler;
 import '../dart_types.dart';
@@ skipping 23 matching lines @@
   IDENTICAL,
   INT,
   INT_FROM_ENVIRONMENT,
   LIST,
   MAP,
   NULL,
   STRING,
   STRING_FROM_ENVIRONMENT,
   STRING_LENGTH,
   SYMBOL,
+  SYNTHETIC,
   TYPE,
   UNARY,
   VARIABLE,
 
   POSITIONAL_REFERENCE,
   NAMED_REFERENCE,
 }
 
 /// Environment used for evaluating constant expressions.
 abstract class Environment {
@@ skipping 273 matching lines @@
     return new NonConstantValue();
   }
 
   @override
   int _computeHashCode() => 13;
 
   @override
   bool _equals(ErroneousConstantExpression other) => true;
 }
 
+// TODO(johnniwinther): Avoid the need for this class.
+class SyntheticConstantExpression extends ConstantExpression {
+  final SyntheticConstantValue value;
+
+  SyntheticConstantExpression(this.value);
+
+  @override
+  ConstantValue evaluate(Environment environment,
+                         ConstantSystem constantSystem) {
+    return value;
+  }
+
+  @override
+  int _computeHashCode() => 13 * value.hashCode;
+
+  accept(ConstantExpressionVisitor visitor, [context]) {
+    throw "unsupported";
+  }
+
+  @override
+  bool _equals(SyntheticConstantExpression other) {
+    return value == other.value;
+  }
+
+  ConstantExpressionKind get kind => ConstantExpressionKind.SYNTHETIC;
+}
+
+
+
 /// A boolean, int, double, string, or null constant.
 abstract class PrimitiveConstantExpression extends ConstantExpression {
   /// The primitive value of this contant expression.
   get primitiveValue;
 }
 
 /// Boolean literal constant.
 class BoolConstantExpression extends PrimitiveConstantExpression {
   final bool primitiveValue;
 
@@ skipping 1427 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }