Fix Flow return types on arrow functions (#124)

* fix: arrow return type on next line is valid

https://github.com/babel/babel-eslint/issues/348

* fix: arrow on line after return type annotation is invalid

* lint

src/parser/expression.js

@@ -577,7 +577,7 @@ pp.parseParenAndDistinguishExpression = function (startPos, startLoc, canBeArrow
   this.expect(tt.parenR);
 
   let arrowNode = this.startNodeAt(startPos, startLoc);
-  if (canBeArrow && !this.canInsertSemicolon() && (arrowNode = this.parseArrow(arrowNode))) {
+  if (canBeArrow && this.shouldParseArrow() && (arrowNode = this.parseArrow(arrowNode))) {
     for (let param of exprList) {
       if (param.extra && param.extra.parenthesized) this.unexpected(param.extra.parenStart);
     }
@@ -613,6 +613,10 @@ pp.parseParenAndDistinguishExpression = function (startPos, startLoc, canBeArrow
   return val;
 };
 
+pp.shouldParseArrow = function () {
+  return !this.canInsertSemicolon();
+};
+
 pp.parseArrow = function (node) {
   if (this.eat(tt.arrow)) {
     return node;

src/plugins/flow.js

@@ -1178,6 +1178,7 @@ export default function (instance) {
         let state = this.state.clone();
         try {
           let returnType = this.flowParseTypeAnnotation();
+          if (this.canInsertSemicolon()) this.unexpected();
           if (!this.match(tt.arrow)) this.unexpected();
           // assign after it is clear it is an arrow
           node.returnType = returnType;
@@ -1194,6 +1195,12 @@ export default function (instance) {
     };
   });
 
+  instance.extend("shouldParseArrow", function (inner) {
+    return function () {
+      return this.match(tt.colon) || inner.call(this);
+    };
+  });
+
   instance.extend("isClassMutatorStarter", function (inner) {
     return function () {
       if (this.isRelational("<")) {