Actual fix + test

src/compiler/checker.ts

@@ -16377,6 +16377,9 @@ namespace ts {
                     // Note:JS inferred classes might come from a variable declaration instead of a function declaration.
                     // In this case, using getResolvedSymbol directly is required to avoid losing the members from the declaration.
                     let funcSymbol = checkExpression(node.expression).symbol;
+                    if (!funcSymbol && node.expression.kind === SyntaxKind.Identifier) {
+                        funcSymbol = getResolvedSymbol(node.expression as Identifier);
+                    }
                     if (funcSymbol && isDeclarationOfFunctionOrClassExpression(funcSymbol)) {
                         funcSymbol = getSymbolOfNode((<VariableDeclaration>funcSymbol.valueDeclaration).initializer);
                     }

tests/cases/fourslash/indirectClassInstantiation.ts

@@ -0,0 +1,21 @@
+/// <reference path="fourslash.ts" />
+
+// @allowJs: true
+// @Filename: something.js
+//// function TestObj(){
+////     this.property = "value";
+//// }
+//// var constructor = TestObj;
+//// var instance = new constructor();
+//// instance./*a*/
+//// var class2 = function() { };
+//// class2.prototype.blah = function() { };
+//// var inst2 = new class2();
+//// inst2.blah/*b*/;
+
+goTo.marker('a');
+verify.completionListContains('property');
+edit.backspace();
+
+goTo.marker('b');
+verify.quickInfoIs('(property) class2.blah: () => void');