Use predicates to check simple function types.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 965822ea6c0c54cf5ca4370ebb5b6f15ecb59839..f1f7953b31ec098d928e1b9f4e819b9ed0ddde4f 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -2176,7 +2176,9 @@ class CodeEmitterTask extends CompilerTask {
       if (!knownSubtype) {
         registerDynamicFunctionTypeCheck(functionType);
         hasDynamicFunctionTypeCheck = true;
-      } else {
+      } else if (!backend.rti.isSimpleFunctionType(functionType)) {
+        // Simple function types are always checked using predicates and should
+        // not provoke generation of signatures.
         neededPredicates++;
       }
     });
@@ -2188,7 +2190,10 @@ class CodeEmitterTask extends CompilerTask {
     }
     functionTypeChecks.forEach((FunctionType functionType, bool knownSubtype) {
       if (knownSubtype) {
-        if (alwaysUseSignature) {
+        if (backend.rti.isSimpleFunctionType(functionType)) {
+          // Simple function types are always checked using predicates.
+          emitIsFunctionTypeTest(functionType);
+        } else if (alwaysUseSignature) {
           registerDynamicFunctionTypeCheck(functionType);
         } else {
           emitIsFunctionTypeTest(functionType);