Add concise method syntax to javascript fixture grammar

This exposes an ambiguity handling bug that I discovered while adding ES6 support to
tree-sitter-javascript

spec/fixtures/corpus/javascript/control_flow.txt

@@ -30,6 +30,49 @@ if (isReady()) {
   (statement_block (expression_statement
     (function_call (member_access (identifier) (identifier)) (arguments (identifier)))))))
 
+==========================================
+if statements whose bodies are objects
+==========================================
+
+if (a) {
+  b()
+  {
+    c();
+  },
+
+  d: 'e'
+}
+
+---
+
+(program (if_statement (identifier)
+  (expression_statement (object
+    (method_definition (identifier) (statement_block (expression_statement (function_call (identifier) (arguments)))))
+    (pair (identifier) (string))))))
+
+============================================
+if statements whose bodies look like objects
+============================================
+
+if (f) {
+  g()
+  {
+    h();
+    i();
+  }
+  j()
+}
+
+---
+
+(program (if_statement (identifier)
+  (statement_block
+    (expression_statement (function_call (identifier) (arguments)))
+    (statement_block
+      (expression_statement (function_call (identifier) (arguments)))
+      (expression_statement (function_call (identifier) (arguments))))
+    (expression_statement (function_call (identifier) (arguments))))))
+
 ==========================================
 if-else statements
 ==========================================

spec/fixtures/grammars/javascript.cc

@@ -202,7 +202,9 @@ extern const Grammar javascript = Grammar({
     sym("true"),
     sym("_paren_expression") }) },
 
-  { "object", in_braces(comma_sep(err(sym("pair")))) },
+  { "object", in_braces(comma_sep(err(choice({
+    sym("pair"),
+    sym("method_definition") })))) },
 
   { "array", in_brackets(comma_sep(err(sym("_expression")))) },
 
@@ -340,12 +342,20 @@ extern const Grammar javascript = Grammar({
     str(":"),
     sym("_expression") }) },
 
+  { "method_definition", seq({
+    sym("identifier"),
+    str("("),
+    comma_sep(sym("identifier")),
+    str(")"),
+    sym("statement_block") }) },
+
 }).ubiquitous_tokens({
   sym("comment"),
   sym("_line_break"),
   pattern("[ \t\r]"),
 }).expected_conflicts({
   { "for_in_statement", "_expression" },
+  { "method_definition", "_expression" },
 });
 
 }  // namespace tree_sitter_examples