Fix bugs for go-to-definition on mapped type method (microsoft#25991)

Author: Andy

src/services/goToDefinition.ts

@@ -34,12 +34,12 @@ namespace ts.GoToDefinition {
             // For a function, if this is the original function definition, return just sigInfo.
             // If this is the original constructor definition, parent is the class.
             if (typeChecker.getRootSymbols(symbol).some(s => symbolMatchesSignature(s, calledDeclaration)) ||
-                // TODO: GH#23742 Following check shouldn't be necessary if 'require' is an alias
-                symbol.declarations.some(d => isVariableDeclaration(d) && !!d.initializer && isRequireCall(d.initializer, /*checkArgumentIsStringLiteralLike*/ false))) {
+                // TODO: GH#25533 Following check shouldn't be necessary if 'require' is an alias
+                symbol.declarations && symbol.declarations.some(d => isVariableDeclaration(d) && !!d.initializer && isRequireCall(d.initializer, /*checkArgumentIsStringLiteralLike*/ false))) {
                 return [sigInfo];
             }
             else {
-                const defs = getDefinitionFromSymbol(typeChecker, symbol, node)!;
+                const defs = getDefinitionFromSymbol(typeChecker, symbol, node) || emptyArray;
                 // For a 'super()' call, put the signature first, else put the variable first.
                 return node.kind === SyntaxKind.SuperKeyword ? [sigInfo, ...defs] : [...defs, sigInfo];
             }

tests/cases/fourslash/goToDefinition_mappedType.ts

@@ -0,0 +1,7 @@
+///<reference path="fourslash.ts"/>
+
+////interface I { /*def*/m(): void; };
+////declare const i: { [K in "m"]: I[K] };
+////i.[|/*ref*/m|]();
+
+verify.goToDefinition("ref", "def");