[scanner/parser] Naturally avoid bounds check in SimpleToken.type

By making the constant array have size 256 we "naturally" avoid the
bounds check in SimpleToken.type.

Stats for 10 runs each before/after:
```
page-faults:u: -0.1222% +/- 0.0686% (-127.40 +/- 71.50)
instructions:u: -0.2589% +/- 0.0007% (-55273529.20 +/- 147748.67)
```

Stats for 100 runs each before/after:
```
msec task-clock:u: -1.6562% +/- 0.7810% (-71.89 +/- 33.90)
page-faults:u: -0.1251% +/- 0.0186% (-130.44 +/- 19.36)
cycles:u: -1.6881% +/- 0.8129% (-302264895.65 +/- 145560853.90)
instructions:u: -0.2593% +/- 0.0002% (-55345822.71 +/- 36070.85)
branch-misses:u: -5.9592% +/- 2.5980% (-4206550.69 +/- 1833947.73)
seconds time elapsed: -1.6470% +/- 0.7804% (-0.07 +/- 0.03)
seconds user: -1.6494% +/- 0.8506% (-0.07 +/- 0.04)
```

To my surprise this also reduced the size of the aot compiled CFE
by ~20 kb.

Change-Id: I149eda23a0b88f06953156021389bd85728e97ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387920
Commit-Queue: Jens Johansen <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: jensjoha

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

@@ -546,7 +546,7 @@ class SimpleToken implements Token {
    */
   @override
   void set offset(int value) {
-    assert(_tokenTypesByIndex.length < 256);
+    assert(_tokenTypesByIndex.length == 256);
     // See https://github.com/dart-lang/sdk/issues/50048 for details.
     assert(value >= -1);
     _typeAndOffset = ((value + 1) << 8) | (_typeAndOffset & 0xff);
@@ -580,7 +580,7 @@ class SimpleToken implements Token {
     assert(offset >= -1);
 
     // Assert the encoding of the [type] is fully reversible.
-    assert(type.index < 256 && _tokenTypesByIndex.length < 256);
+    assert(type.index < 256 && _tokenTypesByIndex.length == 256);
     assert(identical(offset, this.offset));
     assert(identical(type, this.type), '$type != ${this.type}');
 
@@ -1226,6 +1226,9 @@ class TokenClass {
  * Clients may not extend, implement or mix-in this class.
  */
 class TokenType {
+  static const TokenType UNUSED = const TokenType(
+      /* index = */ 255, '', 'UNUSED', NO_PRECEDENCE, EOF_TOKEN);
+
   /**
    * The type of the token that marks the start or end of the input.
    */
@@ -1875,9 +1878,11 @@ class TokenType {
   String toString() => name;
 }
 
-/**
- * Constant list of [TokenType] and [Keyword] ordered by index.
- */
+/// Constant list of [TokenType] and [Keyword] ordered by index.
+///
+/// This list should always have length 256 to avoid bounds checks in
+/// SimpleToken.type:
+/// DartDocTest(_tokenTypesByIndex.length, 256)
 const List<TokenType> _tokenTypesByIndex = [
   TokenType.EOF,
   TokenType.DOUBLE,
@@ -2034,4 +2039,106 @@ const List<TokenType> _tokenTypesByIndex = [
   Keyword.WHILE,
   Keyword.WITH,
   Keyword.YIELD,
+  // Fill to length 256 to avoid bounds check in SimpleToken.type.
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
+  TokenType.UNUSED,
 ];