dart2js cps: Support 'on T' clauses without a 'catch'.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

 // Copyright (c) 2013, 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.ir_builder_task;
 
 import '../closure.dart' as closurelib;
 import '../closure.dart' hide ClosureScope;
 import '../constants/expressions.dart';
 import '../dart_types.dart';
@@ skipping 435 matching lines @@
     JumpTarget target = elements.getTargetDefinition(node);
     Element error =
         (compiler.backend as JavaScriptBackend).getFallThroughError();
     irBuilder.buildSimpleSwitch(target, value, cases, defaultCase, error,
         sourceInformationBuilder.buildGeneric(node));
   }
 
   visitTryStatement(ast.TryStatement node) {
     List<CatchClauseInfo> catchClauseInfos = <CatchClauseInfo>[];
     for (ast.CatchBlock catchClause in node.catchBlocks.nodes) {
-      assert(catchClause.exception != null);
-      LocalVariableElement exceptionVariable = elements[catchClause.exception];
+      LocalVariableElement exceptionVariable;
+      if (catchClause.exception != null) {
+        exceptionVariable = elements[catchClause.exception];
+      }
       LocalVariableElement stackTraceVariable;
       if (catchClause.trace != null) {
         stackTraceVariable = elements[catchClause.trace];
       }
       DartType type;
       if (catchClause.onKeyword != null) {
         type = elements.getType(catchClause.type);
       }
       catchClauseInfos.add(new CatchClauseInfo(
           type: type,
@@ skipping 2881 matching lines @@
     if (compiler.backend.isForeign(function)) {
       return handleForeignCode(node, function, argumentList, callStructure);
     } else {
       return irBuilder.buildStaticFunctionInvocation(function, callStructure,
           translateStaticArguments(argumentList, function, callStructure),
           sourceInformation:
               sourceInformationBuilder.buildCall(node, node.selector));
     }
   }
 }