Adding check or trust type checks to builder_kernel.dart.

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index 35ee5275c819bff665274e2dcc01b321e442e492..a5578f172abd38575f291e90598c164382b75b46 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -250,6 +250,8 @@ class KernelAstAdapter {
     return _backend.isInterceptedSelector(selector);
   }
 
+  LibraryElement get jsHelperLibrary => _backend.helpers.jsHelperLibrary;
+
   JumpTarget getTargetDefinition(ir.Node node) =>
       elements.getTargetDefinition(getNode(node));
 
@@ -293,9 +295,13 @@ class KernelAstAdapter {
     return types.map(getDartType).toList();
   }
 
+  DartType getFunctionReturnType(ir.FunctionNode node) {
+    return getDartType(node.returnType);
+  }
+
   /// Computes the function type corresponding the signature of [node].
   FunctionType getFunctionType(ir.FunctionNode node) {
-    DartType returnType = getDartType(node.returnType);
+    DartType returnType = getFunctionReturnType(node);
     List<DartType> parameterTypes = <DartType>[];
     List<DartType> optionalParameterTypes = <DartType>[];
     for (ir.VariableDeclaration variable in node.positionalParameters) {