Fix invariant breakage on erroneous constant variables.

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 '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 1243 matching lines @@
 
   accept(ConstantExpressionVisitor visitor, [context]) {
     return visitor.visitBoolFromEnvironment(this, context);
   }
 
   @override
   void _createStructuredText(StringBuffer sb) {
     sb.write('bool.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
     ConstantValue nameConstantValue =
         name.evaluate(environment, constantSystem);
     ConstantValue defaultConstantValue;
     if (defaultValue != null) {
@@ skipping 38 matching lines @@
 
   accept(ConstantExpressionVisitor visitor, [context]) {
     return visitor.visitIntFromEnvironment(this, context);
   }
 
   @override
   void _createStructuredText(StringBuffer sb) {
     sb.write('int.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
     ConstantValue nameConstantValue =
         name.evaluate(environment, constantSystem);
     ConstantValue defaultConstantValue;
     if (defaultValue != null) {
@@ skipping 40 matching lines @@
 
   accept(ConstantExpressionVisitor visitor, [context]) {
     return visitor.visitStringFromEnvironment(this, context);
   }
 
   @override
   void _createStructuredText(StringBuffer sb) {
     sb.write('String.fromEnvironment(name=');
     name._createStructuredText(sb);
     sb.write(',defaultValue=');
-    defaultValue._createStructuredText(sb);
+    if (defaultValue != null) {
+      defaultValue._createStructuredText(sb);
+    } else {
+      sb.write('null');
+    }
     sb.write(')');
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
     ConstantValue nameConstantValue =
         name.evaluate(environment, constantSystem);
     ConstantValue defaultConstantValue;
     if (defaultValue != null) {
@@ skipping 365 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }