Fix tokenization of properties named 'class' or 'function'

FIX: Fix an issue where tokenizing (without parsing) an object literal with a property
named `class` or `function` could, in some circumstance, put the tokenizer into an
invalid state.

Closes #1242

acorn/src/tokencontext.js

@@ -79,7 +79,7 @@ pp.updateContext = function(prevType) {
     this.exprAllowed = type.beforeExpr
 }
 
-// Used to handle egde cases when token context could not be inferred correctly during tokenization phase
+// Used to handle edge cases when token context could not be inferred correctly during tokenization phase
 
 pp.overrideContext = function(tokenCtx) {
   if (this.curContext() !== tokenCtx) {
@@ -132,6 +132,13 @@ tt._function.updateContext = tt._class.updateContext = function(prevType) {
   this.exprAllowed = false
 }
 
+tt.colon.updateContext = function() {
+  if (this.curContext().token === "function") {
+    this.context.pop()
+    this.exprAllowed = true
+  }
+}
+
 tt.backQuote.updateContext = function() {
   if (this.curContext() === types.q_tmpl)
     this.context.pop()