Use JavaScript runtime semantics when constant folding.

lib/compiler/implementation/constant_system.dart

Index: lib/compiler/implementation/constant_system.dart
diff --git a/lib/compiler/implementation/constant_system.dart b/lib/compiler/implementation/constant_system.dart
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+// Copyright (c) 2012, 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.
+
+interface Operation {
+  final SourceString name;
+  bool isUserDefinable();
+}
+
+interface UnaryOperation extends Operation {
+  /** Returns [:null:] if it was unable to fold the operation. */
+  Constant fold(Constant constant);
+}
+
+interface BinaryOperation extends Operation {
+  /** Returns [:null:] if it was unable to fold the operation. */
+  Constant fold(Constant left, Constant right);
+}
+
+/**
+ * A [ConstantSystem] is responsible for creating constants and folding them.
+ */
+interface ConstantSystem {
+  BinaryOperation get add();
+  BinaryOperation get bitAnd();
+  UnaryOperation get bitNot();
+  BinaryOperation get bitOr();
+  BinaryOperation get bitXor();
+  BinaryOperation get booleanAnd();
+  BinaryOperation get booleanOr();
+  BinaryOperation get divide();
+  BinaryOperation get equal();
+  BinaryOperation get greaterEqual();
+  BinaryOperation get greater();
+  BinaryOperation get identity();
+  BinaryOperation get lessEqual();
+  BinaryOperation get less();
+  BinaryOperation get modulo();
+  BinaryOperation get multiply();
+  UnaryOperation get negate();
+  UnaryOperation get not();
+  BinaryOperation get shiftLeft();
+  BinaryOperation get shiftRight();
+  BinaryOperation get subtract();
+  BinaryOperation get truncatingDivide();
+
+  Constant createInt(int i);
+  Constant createDouble(double d);
+  // We need a diagnostic node to report errors in case the string is malformed.
+  Constant createString(DartString string, Node diagnosticNode);
+  Constant createBool(bool value);
+  Constant createNull();
+
+  /** Returns true if the [constant] is an integer at runtime. */
+  bool isInt(Constant constant);
+  /** Returns true if the [constant] is a double at runtime. */
+  bool isDouble(Constant constant);
+  /** Returns true if the [constant] is a string at runtime. */
+  bool isString(Constant constant);
+  /** Returns true if the [constant] is a boolean at runtime. */
+  bool isBool(Constant constant);
+  /** Returns true if the [constant] is null at runtime. */
+  bool isNull(Constant constant);
+}