add migration engine support for imported top level var references

In addition to adding imported top level var references support,
this revert GraphBuilder._checkAssignment changes so that the method
conforms to team expectations and addresses comments in
https://dart-review.googlesource.com/c/sdk/+/106562

Change-Id: I3e2b021ec51a1363706a7f6811807d0e6a680df5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106920
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: Dan Rubel

pkg/nnbd_migration/lib/src/decorated_type.dart

@@ -87,6 +87,8 @@ class DecoratedType {
     DecoratedType decoratedType;
     if (element is ExecutableElement) {
       decoratedType = decorate(element.type);
+    } else if (element is TopLevelVariableElement) {
+      decoratedType = decorate(element.type);
     } else {
       // TODO(paulberry)
       throw UnimplementedError('Decorating ${element.runtimeType}');

pkg/nnbd_migration/lib/src/graph_builder.dart

@@ -638,8 +638,12 @@ $stackTrace''');
         staticElement is LocalVariableElement) {
       return getOrComputeElementType(staticElement);
     } else if (staticElement is PropertyAccessorElement) {
-      // TODO(danrubel): assuming getter context... need to handle setter
-      return getOrComputeElementType(staticElement).returnType;
+      if (staticElement.isGetter) {
+        return getOrComputeElementType(staticElement).returnType;
+      } else {
+        // TODO(danrubel) handle setter
+        _unimplemented(node, 'Setter');
+      }
     } else if (staticElement is ClassElement) {
       return _nonNullableTypeType;
     } else {
@@ -738,14 +742,10 @@ $stackTrace''');
         guards: _guards, hard: hard);
     expressionChecks?.edges?.add(edge);
     // TODO(paulberry): generalize this.
-
     if ((_isSimple(sourceType) || destinationType.type.isObject) &&
         _isSimple(destinationType)) {
       // Ok; nothing further to do.
-      return;
-    }
-
-    if (sourceType.type is InterfaceType &&
+    } else if (sourceType.type is InterfaceType &&
         destinationType.type is InterfaceType &&
         sourceType.type.element == destinationType.type.element) {
       assert(sourceType.typeArguments.length ==
@@ -755,16 +755,11 @@ $stackTrace''');
             sourceType.typeArguments[i], expressionChecks,
             hard: false);
       }
-      return;
-    }
-
-    if (destinationType.type.isDynamic || sourceType.type.isDynamic) {
+    } else if (destinationType.type.isDynamic || sourceType.type.isDynamic) {
       // ok; nothing further to do.
-      return;
+    } else {
+      throw '$destinationType <= $sourceType'; // TODO(paulberry)
     }
-
-    // TODO(paulberry)
-    throw '$destinationType <= $sourceType';
   }
 
   /// Double checks that [name] is not the name of a method or getter declared

pkg/nnbd_migration/test/graph_builder_test.dart

@@ -1667,15 +1667,23 @@ int f() {
 double pi = 3.1415;
 double get myPi => pi;
 ''');
-    var pi = findNode.topLevelVariableDeclaration('double pi');
-    var piType =
-        variables.decoratedTypeAnnotation(testSource, pi.variables.type);
-    var myPi = findNode.any('myPi').parent as FunctionDeclaration;
-    var myPiType =
-        variables.decoratedTypeAnnotation(testSource, myPi.returnType);
+    var piType = decoratedTypeAnnotation('double pi');
+    var myPiType = decoratedTypeAnnotation('double get');
     assertEdge(piType.node, myPiType.node, hard: false);
   }
 
+  test_topLevelVar_reference_differentPackage() async {
+    addPackageFile('pkgPi', 'piConst.dart', '''
+double pi = 3.1415;
+''');
+    await analyze('''
+import "package:pkgPi/piConst.dart";
+double get myPi => pi;
+''');
+    var myPiType = decoratedTypeAnnotation('double get');
+    assertEdge(never, myPiType.node, hard: false);
+  }
+
   test_type_argument_explicit_bound() async {
     await analyze('''
 class C<T extends Object> {}