Revert "Support runtime check of function types."

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
index f8ef54e8c134f8f613b17c7e6f0e5c1da1cb5b30..ce291f22e1685b73ec61edb9b7f60d258fddc13b 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
@@ -596,14 +596,8 @@ class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
   HInstruction visitTypeConversion(HTypeConversion node) {
     HInstruction value = node.inputs[0];
     DartType type = node.typeExpression;
-    if (type != null) {
-      if (!type.isRaw || type.kind == TypeKind.TYPE_VARIABLE) {
-        return node;
-      }
-      if (type.kind == TypeKind.FUNCTION) {
-        // TODO(johnniwinther): Optimize function type conversions.
-        return node;
-      }
+    if (type != null && (!type.isRaw || type.kind == TypeKind.TYPE_VARIABLE)) {
+      return node;
     }
     HType convertedType = node.instructionType;
     if (convertedType.isUnknown()) return node;