Throw Error on unreachable code rather than a String.

pkg/compiler/lib/src/js_backend/backend.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 js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 263 matching lines @@
   LibraryElement asyncLibrary;
   LibraryElement interceptorsLibrary;
   LibraryElement foreignLibrary;
   LibraryElement isolateHelperLibrary;
 
   ClassElement closureClass;
   ClassElement boundClosureClass;
   Element assertTestMethod;
   Element assertThrowMethod;
   Element assertHelperMethod;
+  Element assertUnreachableMethod;
   Element invokeOnMethod;
 
   ClassElement jsInterceptorClass;
   ClassElement jsStringClass;
   ClassElement jsArrayClass;
   ClassElement jsNumberClass;
   ClassElement jsIntClass;
   ClassElement jsDoubleClass;
   ClassElement jsNullClass;
   ClassElement jsBoolClass;
@@ skipping 896 matching lines @@
       Element e = findHelper('boolConversionCheck');
       if (e != null) enqueue(world, e, registry);
     }
 
     if (TRACE_CALLS) {
       traceHelper = findHelper(
           TRACE_METHOD == 'console' ? 'consoleTraceHelper' : 'postTraceHelper');
       assert(traceHelper != null);
       enqueueInResolution(traceHelper, registry);
     }
+    enqueueInResolution(assertUnreachableMethod, registry);
     registerCheckedModeHelpers(registry);
   }
 
   onResolutionComplete() {
     super.onResolutionComplete();
     computeMembersNeededForReflection();
     rti.computeClassesNeedingRti();
   }
 
   onTypeInferenceComplete() {
@@ skipping 907 matching lines @@
         jsUnmodifiableArrayClass = findClass('JSUnmodifiableArray');
         jsPlainJavaScriptObjectClass = findClass('PlainJavaScriptObject');
         jsUnknownJavaScriptObjectClass = findClass('UnknownJavaScriptObject');
         jsIndexableClass = findClass('JSIndexable');
         jsMutableIndexableClass = findClass('JSMutableIndexable');
       } else if (uri == DART_JS_HELPER) {
         initializeHelperClasses();
         assertTestMethod = findHelper('assertTest');
         assertThrowMethod = findHelper('assertThrow');
         assertHelperMethod = findHelper('assertHelper');
+        assertUnreachableMethod = findHelper('assertUnreachable');
 
         typeLiteralClass = findClass('TypeImpl');
         constMapLiteralClass = findClass('ConstantMap');
         typeVariableClass = findClass('TypeVariable');
 
         jsIndexingBehaviorInterface = findClass('JavaScriptIndexingBehavior');
 
         noSideEffectsClass = findClass('NoSideEffects');
         noThrowsClass = findClass('NoThrows');
         noInlineClass = findClass('NoInline');
@@ skipping 1033 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }