Disallow assignment to a top level method in dartc

compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java

Index: compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java
diff --git a/compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java b/compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java
index 85e5240710eb20d89e07fe1e1cdee5269e845a02..1421b390168b41474c8445a0b55c86ed7a9713d7 100644
--- a/compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java
+++ b/compiler/javatests/com/google/dart/compiler/resolver/NegativeResolverTest.java
@@ -1074,4 +1074,15 @@ public class NegativeResolverTest extends CompilerTestCase {
         "class MyClass implements Map<Object, Dynamic> {",
         "}"));
   }
+
+  public void testAssignToFunc() {
+    checkSourceErrors(
+        makeCode(
+            "// filler filler filler filler filler filler filler filler filler filler",
+            "double func(a) {}",
+            "main() {",
+            "  func = null;",
+            "}"),
+         errEx(ResolverErrorCode.CANNOT_ASSIGN_TO_METHOD, 4, 3, 4));
+  }
 }