Add ConstantConstructor to ConstantExpression system.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 4671212ffad60e10fdac870bc96917479810655b..cfe245c0e9a4c0396576f2acdf4d4f90a8251513 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -852,9 +852,15 @@ abstract class Compiler implements DiagnosticListener {
   Element identicalFunction;
   Element loadLibraryFunction;
   Element functionApplyMethod;
-  Element intEnvironment;
-  Element boolEnvironment;
-  Element stringEnvironment;
+
+  /// The [int.fromEnvironment] constructor.
+  ConstructorElement intEnvironment;
+
+  /// The [bool.fromEnvironment] constructor.
+  ConstructorElement boolEnvironment;
+
+  /// The [String.fromEnvironment] constructor.
+  ConstructorElement stringEnvironment;
 
   /// Tracks elements with compile-time errors.
   final Set<Element> elementsWithCompileTimeErrors = new Set<Element>();