Migration: handle references to already-migrated fields.

This should address ~54 exceptions having this line in the stack trace:

Variables._createDecoratedElementType (package:nnbd_migration/src/variables.dart:241:7)

Note: this change exposed issue flutter#38257, so it includes a workaround for it.
Change-Id: Id533c070a02d310661a05035679d9476ad2ab240
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116220
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/nnbd_migration/lib/src/nullability_migration_impl.dart

@@ -39,7 +39,9 @@ class NullabilityMigrationImpl implements NullabilityMigration {
   void finish() {
     _graph.propagate();
     if (_graph.unsatisfiedSubstitutions.isNotEmpty) {
-      throw new UnimplementedError('Need to report unsatisfied substitutions');
+      // TODO(paulberry): for now we just ignore unsatisfied substitutions, to
+      // work around https://github.com/dart-lang/sdk/issues/38257
+      // throw new UnimplementedError('Need to report unsatisfied substitutions');
     }
     // TODO(paulberry): it would be nice to report on unsatisfied edges as well,
     // however, since every `!` we add has an unsatisfied edge associated with

pkg/nnbd_migration/lib/src/variables.dart

@@ -232,9 +232,9 @@ class Variables implements VariableRecorder, VariableRepository {
     }
 
     DecoratedType decoratedType;
-    if (element is ExecutableElement) {
+    if (element is FunctionTypedElement) {
       decoratedType = _alreadyMigratedCodeDecorator.decorate(element.type);
-    } else if (element is TopLevelVariableElement) {
+    } else if (element is VariableElement) {
       decoratedType = _alreadyMigratedCodeDecorator.decorate(element.type);
     } else {
       // TODO(paulberry)

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -352,6 +352,16 @@ class EdgeBuilderTest extends EdgeBuilderTestBase {
     return variables.decoratedExpressionType(findNode.expression(text));
   }
 
+  test_already_migrated_field() async {
+    await analyze('''
+double f() => double.NAN;
+''');
+    var nanElement = typeProvider.doubleType.element.getField('NAN');
+    assertEdge(variables.decoratedElementType(nanElement).node,
+        decoratedTypeAnnotation('double f').node,
+        hard: false);
+  }
+
   test_assert_demonstrates_non_null_intent() async {
     await analyze('''
 void f(int i) {