Reapply Issue 10911006.

lib/compiler/implementation/resolver.dart

Index: lib/compiler/implementation/resolver.dart
diff --git a/lib/compiler/implementation/resolver.dart b/lib/compiler/implementation/resolver.dart
index 4b6a8f49fdbe5ed61d7e91227689569694408830..7207b165559a76b8199247111649b2590d98d002 100644
--- a/lib/compiler/implementation/resolver.dart
+++ b/lib/compiler/implementation/resolver.dart
@@ -952,6 +952,7 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   final Element enclosingElement;
   final TypeResolver typeResolver;
   bool inInstanceContext;
+  bool inCheckContext;
   Scope scope;
   ClassElement currentClass;
   bool typeRequired = false;
@@ -966,12 +967,12 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
       // fields.
       inInstanceContext = (element.isInstanceMember() && !element.isField())
           || element.isGenerativeConstructor(),
-      this.currentClass = element.isMember() ?
-                              element.getEnclosingClass() :
-                              null,
+      this.currentClass = element.isMember() ? element.getEnclosingClass()
+                                             : null,
       this.statementScope = new StatementScope(),
       typeResolver = new TypeResolver(compiler),
       scope = element.buildEnclosingScope(),
+      inCheckContext = compiler.enableTypeAssertions,
       super(compiler) {
   }
 
@@ -997,6 +998,14 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
     return element;
   }
 
+  doInCheckContext(action()) {
+    bool wasInCheckContext = inCheckContext;
+    inCheckContext = true;
+    var result = action();
+    inCheckContext = wasInCheckContext;
+    return result;
+  }
+
   inStaticContext(action()) {
     bool wasInstanceContext = inInstanceContext;
     inInstanceContext = false;
@@ -1037,7 +1046,12 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
 
   Element visitTypeAnnotation(TypeAnnotation node) {
     DartType type = resolveTypeAnnotation(node);
-    if (type !== null) return type.element;
+    if (type !== null) {
+      if (inCheckContext) {
+        compiler.enqueuer.resolution.registerIsCheck(type);
+      }
+      return type.element;
+    }
     return null;
   }
 
@@ -1347,7 +1361,10 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
       String operatorString = node.selector.asOperator().source.stringValue;
       if (operatorString === 'is' || operatorString === 'as') {
         assert(node.arguments.tail.isEmpty());
-        resolveTypeTest(node.arguments.head);
+        DartType type = resolveTypeTest(node.arguments.head);
+        if (type != null) {
+          compiler.enqueuer.resolution.registerIsCheck(type);
+        }
         resolvedArguments = true;
       } else if (operatorString === '?') {
         Element parameter = mapping[node.receiver];
@@ -1608,9 +1625,13 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
 
   DartType resolveTypeAnnotation(TypeAnnotation node) {
     Function report = typeRequired ? error : warning;
-    return typeResolver.resolveTypeAnnotation(node, inScope: scope,
-                                              onFailure: report,
-                                              whenResolved: useType);
+    DartType type = typeResolver.resolveTypeAnnotation(node, inScope: scope,
+                                                       onFailure: report,
+                                                       whenResolved: useType);
+    if (inCheckContext && type != null) {
+      compiler.enqueuer.resolution.registerIsCheck(type);
+    }
+    return type;
   }
 
   visitModifiers(Modifiers node) {
@@ -1885,7 +1906,7 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
     }
 
     Scope blockScope = new BlockScope(scope);
-    visitIn(node.type, blockScope);
+    doInCheckContext(() => visitIn(node.type, blockScope)); 
     visitIn(node.formals, blockScope);
     visitIn(node.block, blockScope);
   }