Fix host-checked mode error in dart2js by removing bad assertion.

sdk/lib/_internal/compiler/implementation/ssa/builder.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 ssa;
 
 /**
  * A special element for the extra parameter taken by intercepted
  * methods. We need to override [Element.computeType] because our
  * optimizers may look at its declared type.
@@ skipping 1927 matching lines @@
     }
     assert(!current.isClosed());
     if (!stack.isEmpty) compiler.cancel('non-empty instruction stack');
   }
 
   visitClassNode(ClassNode node) {
     compiler.internalError('visitClassNode should not be called', node: node);
   }
 
   visitExpressionStatement(ExpressionStatement node) {
-    assert(isReachable);
+    if (!isReachable) return;
     Throw throwExpression = node.expression.asThrow();
     if (throwExpression != null && inliningStack.isEmpty) {
       visit(throwExpression.expression);
       handleInTryStatement();
       closeAndGotoExit(new HThrow(pop()));
     } else {
       visit(node.expression);
       pop();
     }
   }
@@ skipping 3421 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }