perf: Simplify reading of tokens.

This avoids checking isIdentifierStart for every single token

packages/babel-parser/src/plugins/flow.js

@@ -1933,7 +1933,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     // ensure that inside flow types, we bypass the jsx parser plugin
-    readToken(code: number): void {
+    getTokenFromCode(code: number): void {
       const next = this.state.input.charCodeAt(this.state.pos + 1);
       if (
         this.state.inType &&
@@ -1944,7 +1944,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         this.state.isIterator = true;
         return super.readWord();
       } else {
-        return super.readToken(code);
+        return super.getTokenFromCode(code);
       }
     }
 

packages/babel-parser/src/plugins/jsx/index.js

@@ -93,7 +93,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
                 ++this.state.pos;
                 return this.finishToken(tt.jsxTagStart);
               }
-              return this.getTokenFromCode(ch);
+              return super.getTokenFromCode(ch);
             }
             out += this.state.input.slice(chunkStart, this.state.pos);
             return this.finishToken(tt.jsxText, out);
@@ -520,8 +520,8 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
-    readToken(code: number): void {
-      if (this.state.inPropertyName) return super.readToken(code);
+    getTokenFromCode(code: number): void {
+      if (this.state.inPropertyName) return super.getTokenFromCode(code);
 
       const context = this.curContext();
 
@@ -557,7 +557,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         return this.finishToken(tt.jsxTagStart);
       }
 
-      return super.readToken(code);
+      return super.getTokenFromCode(code);
     }
 
     updateContext(prevType: TokenType): void {

packages/babel-parser/src/plugins/typescript.js

@@ -2213,11 +2213,11 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     // ensure that inside types, we bypass the jsx parser plugin
-    readToken(code: number): void {
+    getTokenFromCode(code: number): void {
       if (this.state.inType && (code === 62 || code === 60)) {
         return this.finishOp(tt.relational, 1);
       } else {
-        return super.readToken(code);
+        return super.getTokenFromCode(code);
       }
     }
 

packages/babel-parser/src/tokenizer/index.js

@@ -214,17 +214,7 @@ export default class Tokenizer extends LocationParser {
     if (curContext.override) {
       curContext.override(this);
     } else {
-      this.readToken(this.state.input.codePointAt(this.state.pos));
-    }
-  }
-
-  readToken(code: number): void {
-    // Identifier or keyword. '\uXXXX' sequences are allowed in
-    // identifiers, so '\' also dispatches to that.
-    if (isIdentifierStart(code) || code === charCodes.backslash) {
-      this.readWord();
-    } else {
-      this.getTokenFromCode(code);
+      this.getTokenFromCode(this.state.input.codePointAt(this.state.pos));
     }
   }
 
@@ -826,6 +816,16 @@ export default class Tokenizer extends LocationParser {
       case charCodes.numberSign:
         this.readToken_numberSign();
         return;
+
+      case charCodes.backslash:
+        this.readWord();
+        return;
+
+      default:
+        if (isIdentifierStart(code)) {
+          this.readWord();
+          return;
+        }
     }
 
     this.raise(