Add beforeSynthetic field for synthetic ")", "]", and "}" tokens

This CL updates SyntheticToken to have a new beforeSynthetic field
and the fasta scanner to set that field for synthetic ")", "]", and "}" tokens.
This makes it much more efficient for the parser to move a synthetic
closer when the scanner has inserted it in a less than optimal location
in the token stream.

This CL also addresses a comment in
Address comment in https://dart-review.googlesource.com/c/sdk/+/51561

Change-Id: I7f056ac25f2ca4cf5edb0ef4bce7f1e9dd76de9a
Reviewed-on: https://dart-review.googlesource.com/52040
Commit-Queue: Dan Rubel <danrubel@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/front_end/lib/src/fasta/parser/parser.dart

@@ -770,8 +770,7 @@ class Parser {
       next.setNext(leftParen);
       leftParen = openParen;
     }
-    token = leftParen;
-    token = parseDottedName(token);
+    token = parseDottedName(leftParen);
     Token next = token.next;
     Token equalitySign;
     if (optional('==', next)) {

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

@@ -361,7 +361,7 @@ abstract class ArrayBasedScanner extends AbstractScanner {
     //      v
     //     EOF
     TokenType type = closeBraceInfoFor(begin);
-    appendToken(new SyntheticToken(type, tokenStart));
+    appendToken(new SyntheticToken(type, tokenStart)..beforeSynthetic = tail);
     begin.endGroup = tail;
     appendErrorToken(new UnmatchedToken(begin));
   }

pkg/front_end/lib/src/scanner/token.dart

@@ -492,6 +492,14 @@ class SimpleToken implements Token {
   @override
   int get charEnd => end;
 
+  @override
+  Token get beforeSynthetic => null;
+
+  @override
+  set beforeSynthetic(Token previous) {
+    // ignored
+  }
+
   @override
   int get end => offset + length;
 
@@ -581,6 +589,7 @@ class SimpleToken implements Token {
   Token setNext(Token token) {
     next = token;
     token.previous = this;
+    token.beforeSynthetic = this;
     return token;
   }
 
@@ -711,6 +720,12 @@ class SyntheticStringToken extends StringToken {
 class SyntheticToken extends SimpleToken {
   SyntheticToken(TokenType type, int offset) : super(type, offset);
 
+  @override
+  Token beforeSynthetic;
+
+  @override
+  bool get isSynthetic => true;
+
   @override
   int get length => 0;
 
@@ -757,6 +772,18 @@ abstract class Token implements SyntacticEntity {
    */
   int get charEnd;
 
+  /**
+   * The token before this synthetic token,
+   * or `null` if this is not a synthetic `)`, `]`, `}`, or `>` token.
+   */
+  Token get beforeSynthetic;
+
+  /**
+   * Set token before this synthetic `)`, `]`, `}`, or `>` token,
+   * and ignored otherwise.
+   */
+  set beforeSynthetic(Token previous);
+
   @override
   int get end;
 

pkg/front_end/test/scanner_fasta_test.dart

@@ -727,7 +727,18 @@ class ScannerTest_Fasta_Direct extends ScannerTest_Fasta_Base {
 
   @override
   Token scan(String source) {
-    return createScanner(source, includeComments: true).tokenize();
+    final Token first = createScanner(source, includeComments: true).tokenize();
+    Token token = first;
+    while (!token.isEof) {
+      Token next = token.next;
+      expect(token.next, next);
+      expect(next.previous, token);
+      if (next.isSynthetic && [')', ']', '}'].contains(next.lexeme)) {
+        expect(next.beforeSynthetic, token);
+      }
+      token = next;
+    }
+    return first;
   }
 
   void test_linestarts() {