Disable scanning of &&= and ||= assignment operators

Fix #30340

Change-Id: Id27f37af9805ca09deaa3c6252817e3d9a926216
Reviewed-on: https://dart-review.googlesource.com/17361
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: Dan Rubel

pkg/analyzer/test/generated/parser_test.dart

@@ -17,6 +17,7 @@ import 'package:analyzer/src/generated/parser.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/testing/token_factory.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
+import 'package:front_end/src/fasta/scanner/abstract_scanner.dart';
 import 'package:front_end/src/scanner/scanner.dart' as fe;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ -5628,13 +5629,15 @@ abstract class ExpressionParserTestMixin implements AbstractParserTestCase {
   }
 
   void test_parseExpression_assign_compound() {
-    enableLazyAssignmentOperators = true;
-    Expression expression = parseExpression('x ||= y');
-    var assignmentExpression = expression as AssignmentExpression;
-    expect(assignmentExpression.leftHandSide, isNotNull);
-    expect(assignmentExpression.operator, isNotNull);
-    expect(assignmentExpression.operator.type, TokenType.BAR_BAR_EQ);
-    expect(assignmentExpression.rightHandSide, isNotNull);
+    if (usingFastaParser && AbstractScanner.LAZY_ASSIGNMENT_ENABLED) {
+      enableLazyAssignmentOperators = true;
+      Expression expression = parseExpression('x ||= y');
+      var assignmentExpression = expression as AssignmentExpression;
+      expect(assignmentExpression.leftHandSide, isNotNull);
+      expect(assignmentExpression.operator, isNotNull);
+      expect(assignmentExpression.operator.type, TokenType.BAR_BAR_EQ);
+      expect(assignmentExpression.rightHandSide, isNotNull);
+    }
   }
 
   void test_parseExpression_comparison() {

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

@@ -32,6 +32,14 @@ import 'token_constants.dart';
 import 'characters.dart';
 
 abstract class AbstractScanner implements Scanner {
+  /**
+   * A flag indicating whether character sequences `&&=` and `||=`
+   * should be tokenized as the assignment operators
+   * [AMPERSAND_AMPERSAND_EQ_TOKEN] and [BAR_BAR_EQ_TOKEN] respectively.
+   * See issue https://github.com/dart-lang/sdk/issues/30340
+   */
+  static const bool LAZY_ASSIGNMENT_ENABLED = false;
+
   final bool includeComments;
 
   /**
@@ -510,7 +518,7 @@ abstract class AbstractScanner implements Scanner {
     next = advance();
     if (identical(next, $BAR)) {
       next = advance();
-      if (identical(next, $EQ)) {
+      if (LAZY_ASSIGNMENT_ENABLED && identical(next, $EQ)) {
         appendPrecedenceToken(TokenType.BAR_BAR_EQ);
         return advance();
       }
@@ -530,7 +538,7 @@ abstract class AbstractScanner implements Scanner {
     next = advance();
     if (identical(next, $AMPERSAND)) {
       next = advance();
-      if (identical(next, $EQ)) {
+      if (LAZY_ASSIGNMENT_ENABLED && identical(next, $EQ)) {
         appendPrecedenceToken(TokenType.AMPERSAND_AMPERSAND_EQ);
         return advance();
       }

pkg/front_end/test/precedence_info_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:front_end/src/fasta/scanner/abstract_scanner.dart'
+    show AbstractScanner;
 import 'package:front_end/src/fasta/scanner/string_scanner.dart';
 import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
 import 'package:front_end/src/scanner/token.dart';
@@ -35,8 +37,10 @@ class PrecedenceInfoTest {
     assertLexeme('#!/'); // SCRIPT_TAG
     assertLexeme('"foo"'); // STRING
     assertLexeme('bar'); // IDENTIFIER
-    assertLexeme('&&=');
-    assertLexeme('||=');
+    if (AbstractScanner.LAZY_ASSIGNMENT_ENABLED) {
+      assertLexeme('&&=');
+      assertLexeme('||=');
+    }
   }
 
   void test_isOperator() {

pkg/front_end/test/scanner_test.dart

@@ -4,6 +4,8 @@
 
 import 'package:front_end/src/base/errors.dart';
 import 'package:front_end/src/base/jenkins_smi_hash.dart';
+import 'package:front_end/src/fasta/scanner/abstract_scanner.dart'
+    show AbstractScanner;
 import 'package:front_end/src/scanner/errors.dart';
 import 'package:front_end/src/scanner/reader.dart';
 import 'package:front_end/src/scanner/scanner.dart';
@@ -153,8 +155,10 @@ abstract class ScannerTestBase {
   }
 
   void test_ampersand_ampersand_eq() {
-    _assertToken(TokenType.AMPERSAND_AMPERSAND_EQ, "&&=",
-        lazyAssignmentOperators: true);
+    if (AbstractScanner.LAZY_ASSIGNMENT_ENABLED) {
+      _assertToken(TokenType.AMPERSAND_AMPERSAND_EQ, "&&=",
+          lazyAssignmentOperators: true);
+    }
   }
 
   void test_ampersand_eq() {
@@ -211,7 +215,9 @@ abstract class ScannerTestBase {
   }
 
   void test_bar_bar_eq() {
-    _assertToken(TokenType.BAR_BAR_EQ, "||=", lazyAssignmentOperators: true);
+    if (AbstractScanner.LAZY_ASSIGNMENT_ENABLED) {
+      _assertToken(TokenType.BAR_BAR_EQ, "||=", lazyAssignmentOperators: true);
+    }
   }
 
   void test_bar_eq() {