Use the correct source when skipping trivia

A custom `SourceMapSource` can optionally provide its own `skipTrivia`
function. If this is not provided then the compiler will use the default
function designed for TypeScript source files.

Previously, when calling this default function we were passing the current
`sourceMapSource` rather than the specified `source` whose trivia needs
to be skipped. This resulted in the `pos` being incorrectly calculated for
external source files that need mapping.

**Side note:**

There are actually two possible constructors available for creating
`SourceMapSource` objects. One of them defaults to an identity function
for the `skipTrivia` function if it is not provided (see
https://github.com/Microsoft/TypeScript/blob/49689894d747714d6b2b8461b9033020efec9625/src/compiler/utilities.ts#L6972-L6976)
and the other one leaves the `skipTrivia` field `undefined` (see
https://github.com/Microsoft/TypeScript/blob/5fc8f1dd801dbacfe7e2d624f80b7a6a3868d180/src/services/services.ts#L776-L797)

Unfortunately, it appears that the second of these two constructors is the
one available when importing the "typescript" module in node.js code.

Author: petebacondarwin

src/compiler/emitter.ts

@@ -4372,10 +4372,10 @@ namespace ts {
         }
 
         /**
-         * Skips trivia such as comments and white-space that can optionally overriden by the source map source
+         * Skips trivia such as comments and white-space that can optionally overridden by the source map source
          */
         function skipSourceTrivia(source: SourceMapSource, pos: number): number {
-            return source.skipTrivia ? source.skipTrivia(pos) : skipTrivia(sourceMapSource.text, pos);
+            return source.skipTrivia ? source.skipTrivia(pos) : skipTrivia(source.text, pos);
         }
 
         /**

src/testRunner/unittests/customTransforms.ts

@@ -129,5 +129,34 @@ namespace ts {
             },
             { sourceMap: true }
         );
+
+        emitsCorrectly("skipTriviaExternalSourceFiles",
+            [
+                {
+                    file: "source.ts",
+                    // The source file contains preceding trivia (e.g. whitespace) to try to confuse the `skipSourceTrivia` function.
+                    text: "         original;"
+                },
+            ],
+            {
+                before: [
+                    context => node => visitNode(node, function visitor(node: Node): Node {
+                        if (isIdentifier(node) && node.text === "original") {
+                            const newNode = createIdentifier("changed");
+                            setSourceMapRange(newNode, {
+                              pos: 0,
+                              end: 7,
+                              // Do not provide a custom skipTrivia function for `source`.
+                              source: createSourceMapSource("another.html", "changed;")
+                            });
+                            return newNode;
+                        }
+                        return visitEachChild(node, visitor, context);
+                    })
+                ]
+            },
+            { sourceMap: true }
+        );
+
     });
 }