Quick work around dart2js crashes

dart/pkg/compiler/lib/src/compile_time_constants.dart

 // 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.
 
 part of dart2js;
 
 /// A [ConstantEnvironment] provides access for constants compiled for variable
 /// initializers.
 abstract class ConstantEnvironment {
   /// Returns the constant for the initializer of [element].
@@ skipping 592 matching lines @@
   AstConstant visitConditional(Conditional node) {
     AstConstant condition = evaluate(node.condition);
     if (condition == null) {
       return null;
     } else if (!condition.value.isBool) {
       DartType conditionType = condition.value.getType(compiler.coreTypes);
       if (isEvaluatingConstant) {
         compiler.reportError(
             node.condition, MessageKind.NOT_ASSIGNABLE,
             {'fromType': conditionType, 'toType': compiler.boolClass.rawType});
+        // TODO(ahe): Don't throw, recover from error.
+        throw new CompilerCancelledException(null);
       }
       return null;
     }
     AstConstant thenExpression = evaluate(node.thenExpression);
     AstConstant elseExpression = evaluate(node.elseExpression);
     if (thenExpression == null || elseExpression == null) {
       return null;
     }
     BoolConstantValue boolCondition = condition.value;
     return new AstConstant(
@@ skipping 476 matching lines @@
     return new AstConstant(
         element,
         element.initializer != null ? element.initializer : element.node,
         constant);
   }
 
   ConstantValue get value => expression.value;
 
   String toString() => expression.toString();
 }