[vm] Tree shake stub targets

Stub target references for member signatures, forwarding stubs and mixin
stubs should be removed when tree shaking. Otherwise we might end up with
dangling references if the stub target was tree shaken.

Closes #44716

TEST=existing tests

Change-Id: I5fd67c0ab8db588ecbe5c40101bbea662e1ffb51
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/180141
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: johnniwinther

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -1326,9 +1326,20 @@ class _TreeShakerPass2 extends Transformer {
           _makeUnreachableBody(node.function);
         }
         node.function.asyncMarker = AsyncMarker.Sync;
-        if (node.concreteForwardingStubTarget != null ||
-            node.abstractForwardingStubTarget != null) {
-          node.stubTarget = null;
+        switch (node.stubKind) {
+          case ProcedureStubKind.Regular:
+          case ProcedureStubKind.NoSuchMethodForwarder:
+            break;
+          case ProcedureStubKind.MemberSignature:
+          case ProcedureStubKind.AbstractForwardingStub:
+          case ProcedureStubKind.ConcreteForwardingStub:
+          case ProcedureStubKind.AbstractMixinStub:
+          case ProcedureStubKind.ConcreteMixinStub:
+            // Make the stub look like a regular procedure so the stub target
+            // isn't expected to be non-null, for instance by the verifier.
+            node.stubKind = ProcedureStubKind.Regular;
+            node.stubTarget = null;
+            break;
         }
         Statistics.methodBodiesDropped++;
       } else if (node is Field) {

pkg/vm/test/modular_kernel_plus_aot_test.dart

@@ -157,8 +157,8 @@ class Foo {
 }
 class Mixin {
   void set mixinProperty(v) {}
-  Foo get mixinProperty{}
-  Foo mixinMethod(v) {}
+  Foo get mixinProperty => new Foo();
+  Foo mixinMethod(v) => new Foo();
 }
 class A1 extends Object with Mixin { }
 class A2 extends Object with Mixin { }

pkg/vm/testcases/transformations/type_flow/transformer/invalidation_cycle.dart.expect

@@ -22,7 +22,7 @@ abstract class Stream extends core::Object {
     : super core::Object::•()
     ;
 [@vm.procedure-attributes.metadata=methodOrSetterCalledDynamically:false,getterCalledDynamically:false,hasThisUses:false,hasNonThisUses:false,hasTearOffUses:false,methodOrSetterSelectorId:1,getterSelectorId:2]  abstract method foobar() → self::StreamSubscription*;
-[@vm.procedure-attributes.metadata=methodOrSetterCalledDynamically:false,getterCalledDynamically:false,hasThisUses:false,hasNonThisUses:false,hasTearOffUses:false,methodOrSetterSelectorId:3]  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+[@vm.procedure-attributes.metadata=methodOrSetterCalledDynamically:false,getterCalledDynamically:false,hasThisUses:false,hasNonThisUses:false,hasTearOffUses:false,methodOrSetterSelectorId:3]  abstract operator ==(dynamic other) → core::bool*;
 }
 abstract class _StreamImpl extends self::Stream {
   synthetic constructor •() → self::_StreamImpl*