[parser] Don't use late variables in the scanner

The usage of late variables become an access, a comparison to a marker,
and a conditional jump.

Having it be non-null and non-late it's just an access.

Running aot compiled scanner benchmarks on
pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart with `--bytes`
gives:

```
    N           Min           Max        Median           Avg        Stddev
x  25     149.49816     154.67461     153.12174     153.03484     1.2430919
+  25     153.82654     157.77314     156.45323     156.15451     1.1301418
Difference at 95.0% confidence
        3.11967 +/- 0.675708
        2.03853% +/- 0.441538%
        (Student's t, pooled s = 1.18796)
```

And running the benchmarker with `--cache --silent --iterations=50` on
the scanner benchmark with the same parameters gives:

```
Comparing snapshot 1 with snapshot 2
msec task-clock:u: -1.9158% +/- 0.3843% (-48.87 +/- 9.80)
cycles:u: -1.9652% +/- 0.3719% (-218241604.28 +/- 41302487.04)
instructions:u: -0.4735% +/- 0.0000% (-116299496.56 +/- 182.39)
seconds time elapsed: -1.9132% +/- 0.3834% (-0.05 +/- 0.01)
seconds user: -2.0074% +/- 0.4009% (-0.05 +/- 0.01)

Comparing snapshot 1 with snapshot 2
msec task-clock:u: -2.1237% +/- 0.2726% (-54.21 +/- 6.96)
L1-icache-load-misses: -6.2140% +/- 3.3961% (-268165.40 +/- 146560.98)
LLC-loads: -0.5702% +/- 0.4680% (-6613.36 +/- 5428.54)
seconds time elapsed: -2.1219% +/- 0.2728% (-0.05 +/- 0.01)
seconds user: -2.0997% +/- 0.2793% (-0.05 +/- 0.01)
```

Change-Id: I38e076a2e8442e3313075eaa600114f664eb415d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411640
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/_fe_analyzer_shared/lib/src/scanner/abstract_scanner.dart

@@ -99,17 +99,17 @@ abstract class AbstractScanner implements Scanner {
    * is not exposed to clients of the scanner, which are expected to invoke
    * [firstToken] to access the token stream.
    */
-  final Token tokens = new Token.eof(/* offset = */ -1);
+  final Token tokens;
 
   /**
    * A pointer to the last scanned token.
    */
-  late Token tail;
+  Token tail;
 
   /**
    * A pointer to the last prepended error token.
    */
-  late Token errorTail;
+  Token errorTail;
 
   @override
   bool hasErrors = false;
@@ -140,26 +140,40 @@ abstract class AbstractScanner implements Scanner {
   int recoveryCount = 0;
   final bool allowLazyStrings;
 
-  AbstractScanner(ScannerConfiguration? config, this.includeComments,
-      this.languageVersionChanged,
+  AbstractScanner(ScannerConfiguration? config, bool includeComments,
+      LanguageVersionChanged? languageVersionChanged,
+      {required int numberOfBytesHint, bool allowLazyStrings = true})
+      : this._(config, includeComments, languageVersionChanged,
+            new Token.eof(/* offset = */ -1),
+            numberOfBytesHint: numberOfBytesHint,
+            allowLazyStrings: allowLazyStrings);
+
+  AbstractScanner._(ScannerConfiguration? config, this.includeComments,
+      this.languageVersionChanged, Token newEofToken,
       {required int numberOfBytesHint, this.allowLazyStrings = true})
       : lineStarts = new LineStarts(numberOfBytesHint),
-        inRecoveryOption = false {
-    this.tail = this.tokens;
-    this.errorTail = this.tokens;
+        inRecoveryOption = false,
+        tokens = newEofToken,
+        tail = newEofToken,
+        errorTail = newEofToken {
     this.configuration = config;
   }
 
   AbstractScanner createRecoveryOptionScanner();
 
   AbstractScanner.recoveryOptionScanner(AbstractScanner copyFrom)
+      : this._recoveryOptionScanner(copyFrom, new Token.eof(/* offset = */ -1));
+
+  AbstractScanner._recoveryOptionScanner(
+      AbstractScanner copyFrom, Token newEofToken)
       : lineStarts = [],
         includeComments = false,
         languageVersionChanged = null,
         inRecoveryOption = true,
-        allowLazyStrings = true {
-    this.tail = this.tokens;
-    this.errorTail = this.tokens;
+        allowLazyStrings = true,
+        tokens = newEofToken,
+        tail = newEofToken,
+        errorTail = newEofToken {
     this._enableTripleShift = copyFrom._enableTripleShift;
     this.tokenStart = copyFrom.tokenStart;
     this.groupingStack = copyFrom.groupingStack;