add fasta.scanner support for lazy assignment operators

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

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index 8b88326a6a9b0d4d641889a213011a45969d631c..84a097d74249f614acbdd79368c7bd1fcb871123 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -34,6 +34,12 @@ abstract class AbstractScanner implements Scanner {
   bool scanGenericMethodComments = false;
 
   /**
+   * A flag indicating whether the lazy compound assignment operators '&&=' and
+   * '||=' are enabled.
+   */
+  bool scanLazyAssignmentOperators = false;
+
+  /**
    * The string offset for the next token that will be created.
    *
    * Note that in the [Utf8BytesScanner], [stringOffset] and [scanOffset] values
@@ -486,11 +492,16 @@ abstract class AbstractScanner implements Scanner {
   }
 
   int tokenizeBar(int next) {
-    // | || |=
+    // | || |= ||=
     next = advance();
     if (identical(next, $BAR)) {
+      next = advance();
+      if (scanLazyAssignmentOperators && identical(next, $EQ)) {
+        appendPrecedenceToken(TokenType.BAR_BAR_EQ);
+        return advance();
+      }
       appendPrecedenceToken(TokenType.BAR_BAR);
-      return advance();
+      return next;
     } else if (identical(next, $EQ)) {
       appendPrecedenceToken(TokenType.BAR_EQ);
       return advance();
@@ -501,11 +512,16 @@ abstract class AbstractScanner implements Scanner {
   }
 
   int tokenizeAmpersand(int next) {
-    // && &= &
+    // && &= & &&=
     next = advance();
     if (identical(next, $AMPERSAND)) {
+      next = advance();
+      if (scanLazyAssignmentOperators && identical(next, $EQ)) {
+        appendPrecedenceToken(TokenType.AMPERSAND_AMPERSAND_EQ);
+        return advance();
+      }
       appendPrecedenceToken(TokenType.AMPERSAND_AMPERSAND);
-      return advance();
+      return next;
     } else if (identical(next, $EQ)) {
       appendPrecedenceToken(TokenType.AMPERSAND_EQ);
       return advance();