JS: Add support for index expressions

javascript/ql/lib/semmle/javascript/internal/TypeResolution.qll

@@ -130,6 +130,13 @@ module TypeResolution {
     or
     SummaryTypeTracker::basicLoadStep(object.(AST::ValueNode).flow(),
       member.(AST::ValueNode).flow(), contents)
+    or
+    exists(IndexExpr index |
+      not exists(index.getPropertyName()) and
+      object = index.getBase() and
+      member = index and
+      contents = DataFlow::ContentSet::arrayElement()
+    )
   }
 
   predicate callTarget(InvokeExpr call, Function target) {

javascript/ql/test/library-tests/UnderlyingTypes/indexExpr.ts

@@ -0,0 +1,8 @@
+import * as express from 'express';
+
+interface I {
+    [s: string]: express.Request;
+}
+function t1(obj: I, x: string) {
+    obj[x]; // $ hasUnderlyingType='express'.Request
+}

javascript/ql/test/library-tests/UnderlyingTypes/test.expected

@@ -30,6 +30,7 @@
 | globals.ts:1:13:1:14 | el | HTMLElement |
 | globals.ts:6:13:6:14 | el | HTMLInputElement |
 | globals.ts:9:13:9:15 | req | Express.Request |
+| indexExpr.ts:7:5:7:10 | obj[x] | 'express'.Request |
 | jsdoc.js:7:13:7:15 | req | 'express'.Request |
 | jsdoc.js:13:13:13:15 | res | 'express'.Response |
 | namedImport.ts:3:13:3:15 | req | 'express'.Request |