Allow numeric literal as property name

samples/objectlit.d.ts

@@ -0,0 +1,3 @@
+declare var ObjectType: { name: string; age: number };
+
+declare var NumericKeyObjectType: { 0: number; 1: number; 2.1: number };

samples/objectlit.d.ts.scala

@@ -0,0 +1,23 @@
+
+import scala.scalajs.js
+import js.annotation._
+import js.|
+
+package objectlit {
+
+@js.native
+@JSGlobal
+object ObjectType extends js.Object {
+  var name: String = js.native
+  var age: Double = js.native
+}
+
+@js.native
+@JSGlobal
+object NumericKeyObjectType extends js.Object {
+  var `0`: Double = js.native
+  var `1`: Double = js.native
+  var `2.1`: Double = js.native
+}
+
+}

src/main/scala/org/scalajs/tools/tsimporter/Trees.scala

@@ -111,11 +111,15 @@ object Trees {
 
   case class BooleanLiteral(value: Boolean) extends Literal
 
-  sealed trait NumberLiteral extends Literal
+  sealed trait NumberLiteral extends Literal with PropertyName
 
-  case class IntLiteral(value: Int) extends NumberLiteral
+  case class IntLiteral(value: Int) extends NumberLiteral {
+    override def name = value.toString
+  }
 
-  case class DoubleLiteral(value: Double) extends NumberLiteral
+  case class DoubleLiteral(value: Double) extends NumberLiteral {
+    override def name = value.toString
+  }
 
   case class StringLiteral(value: String) extends Literal with PropertyName {
     override def name = value

src/main/scala/org/scalajs/tools/tsimporter/parser/TSDefParser.scala

@@ -323,7 +323,7 @@ class TSDefParser extends StdTokenParsers with ImplicitConversions {
   })
 
   lazy val propertyName: Parser[PropertyName] =
-    identifier | stringLiteral
+    identifier | stringLiteral | numberLiteral
 
   lazy val stringLiteral: Parser[StringLiteral] =
     stringLit ^^ StringLiteral