Handle nested if statements with alternates in printer

Previously we saw a bug that when we have nested if statements
and an alternate then the generated code may confuse which
if statement the alternate belongs to.

e.g.

```
if (foo) if (bar) bar(); else baz();
```

But this was handled by looking at the consequent
and if it's an if we add a block. However we didn't
handle situations where it's not an if but the last
recursive statement is an if:

```
if (foo) while (bar) if (baz) baz(); else shoosh()
```

This handles it by recurring until we get the last
statement.

packages/babel-generator/src/generators/statements.js

@@ -16,7 +16,7 @@ export function IfStatement(node: Object) {
   this.push(")");
   this.space();
 
-  let needsBlock = node.alternate && t.isIfStatement(node.consequent);
+  let needsBlock = node.alternate && t.isIfStatement(getLastStatement(node.consequent));
   if (needsBlock) {
     this.push("{");
     this.newline();
@@ -38,6 +38,12 @@ export function IfStatement(node: Object) {
   }
 }
 
+// Recursively get the last statement.
+function getLastStatement(statement) {
+  if (!t.isStatement(statement.body)) return statement;
+  return getLastStatement(statement.body);
+}
+
 export function ForStatement(node: Object) {
   this.keyword("for");
   this.push("(");

packages/babel-generator/test/index.js

@@ -16,12 +16,32 @@ suite("generation", function () {
       assert.ok(t.VISITOR_KEYS[type], type + " should not exist");
     });
   });
+});
 
-  test("valid code", function() {
+
+suite("programmatic generation", function() {
+  test("numeric member expression", function() {
     // Should not generate `0.foo`
     var mem = t.memberExpression(t.numericLiteral(60702), t.identifier("foo"));
     new Function(generate.default(mem).code);
   });
+
+  test("nested if statements needs block", function() {
+    var ifStatement = t.ifStatement(
+      t.stringLiteral("top cond"),
+      t.whileStatement(
+        t.stringLiteral("while cond"),
+        t.ifStatement(
+          t.stringLiteral("nested"),
+          t.expressionStatement(t.numericLiteral(1))
+        )
+      ),
+      t.expressionStatement(t.stringLiteral("alt"))
+    );
+
+    var ast = parse(generate.default(ifStatement).code);
+    assert.equal(ast.program.body[0].consequent.type, 'BlockStatement');
+  });
 });
 
 var suites = require("babel-helper-fixtures").default(__dirname + "/fixtures");