Fix analyzer/CFE integration of some more obscure invalid assignment cases.

This CL fixes handling of the following erroneous assignments:
- Compound assignment to postfix increment (e.g. "x++ += y;")
- Null-aware assignment to postfix increment (e.g. "x++ ??= y;")

There is some code duplication in ContextAwareGenerator now; I will
clean that up in a follow-up CL.

Change-Id: If68bc267b67904562bf8c2e705123d04fdf0b69a
Reviewed-on: https://dart-review.googlesource.com/71321
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/test/src/dart/analysis/driver_resolution_test.dart

@@ -972,6 +972,38 @@ void f(num x, int y) {
     assertElement(yRef, findElement.parameter('y'));
   }
 
+  test_assign_to_postfix_increment_compound() async {
+    addTestFile('''
+void f(num x, int y) {
+  x++ += y;
+}
+''');
+    await resolveTestFile();
+
+    var xRef = findNode.simple('x++');
+    assertType(xRef, 'num');
+    assertElement(xRef, findElement.parameter('x'));
+    var yRef = findNode.simple('y;');
+    assertType(yRef, 'int');
+    assertElement(yRef, findElement.parameter('y'));
+  }
+
+  test_assign_to_postfix_increment_null_aware() async {
+    addTestFile('''
+void f(num x, int y) {
+  x++ ??= y;
+}
+''');
+    await resolveTestFile();
+
+    var xRef = findNode.simple('x++');
+    assertType(xRef, 'num');
+    assertElement(xRef, findElement.parameter('x'));
+    var yRef = findNode.simple('y;');
+    assertType(yRef, 'int');
+    assertElement(yRef, findElement.parameter('y'));
+  }
+
   test_assign_to_prefix_increment() async {
     addTestFile('''
 void f(num x, int y) {

pkg/front_end/lib/src/fasta/kernel/expression_generator.dart

@@ -958,7 +958,16 @@ abstract class ContextAwareGenerator implements Generator {
   Expression buildNullAwareAssignment(
       Expression value, DartType type, int offset,
       {bool voidContext: false}) {
-    return makeInvalidWrite(value);
+    var lhs = buildSimpleRead();
+    // The lhs expression needs to have a parent so that type inference can be
+    // applied to it, but it doesn't matter what the parent is because the
+    // lhs expression won't appear in the tree.  So just give it a quick and
+    // dirty parent.
+    new VariableDeclaration.forValue(lhs);
+
+    return new IllegalAssignmentJudgment(value,
+        assignmentOffset: offset, desugared: makeInvalidWrite(value))
+      ..write = lhs;
   }
 
   @override
@@ -968,7 +977,16 @@ abstract class ContextAwareGenerator implements Generator {
       Procedure interfaceTarget,
       bool isPreIncDec: false,
       bool isPostIncDec: false}) {
-    return makeInvalidWrite(value);
+    var lhs = buildSimpleRead();
+    // The lhs expression needs to have a parent so that type inference can be
+    // applied to it, but it doesn't matter what the parent is because the
+    // lhs expression won't appear in the tree.  So just give it a quick and
+    // dirty parent.
+    new VariableDeclaration.forValue(lhs);
+
+    return new IllegalAssignmentJudgment(value,
+        assignmentOffset: offset, desugared: makeInvalidWrite(value))
+      ..write = lhs;
   }
 
   @override