fix: don't remove comments when removing trailing empty returns (#295)

Before this commit, the comment after a removed return statement would be
destroyed:
http://decaffeinate-project.org/repl/#?evaluate=true&code=-%3E%0A%20%20a%20%20%23%20foo%0A%20%20return

Now, the code is a little more careful by deleting up to the start of the line
or the previous semicolon. Also, the code now exits early and doesn't patch the
`return` node or insert a semicolon at the end, since that was causing a
semicolon at the end of the comment.

This also fixes a crash on this type of input that I ran into when using
magic-string with  Rich-Harris/magic-string#89 applied.

src/stages/main/patchers/BlockPatcher.js

@@ -37,14 +37,22 @@ export default class BlockPatcher extends NodePatcher {
     this.statements.forEach(
       (statement, i, statements) => {
         if (i === statements.length - 1 && this.parent instanceof FunctionPatcher) {
-          let previousStatement = statements[i - 1];
           if (statement instanceof ReturnPatcher && !statement.expression) {
+            let removeStart;
+            if (statements.length > 1) {
+              let startOfLineIndex = this.context.sourceTokens.lastIndexOfTokenMatchingPredicate(
+                token => token.type === NEWLINE || token.type === SEMICOLON,
+                statement.outerStartTokenIndex
+              );
+              removeStart = this.sourceTokenAtIndex(startOfLineIndex).start;
+            } else {
+              removeStart = statement.outerStart;
+            }
             this.remove(
-              previousStatement ?
-                previousStatement.outerEnd :
-                statement.outerStart,
+              removeStart,
               statement.outerEnd
             );
+            return;
           }
         }
         if (statement.isSurroundedByParentheses()) {

test/return_test.js

@@ -27,4 +27,32 @@ describe('return', () => {
       () => true ? null : undefined;
     `)
   );
+
+  it('preserves comments when removing trailing empty returns', () =>
+    check(`
+      ->
+        a  # b
+        return
+    `, `
+      (function() {
+        a;  // b
+      });
+    `)
+  );
+
+  it('correctly removes trailing empty returns on the same line as another statement', () =>
+    check(`
+      -> a; return
+    `, `
+      (function() { a; });
+    `)
+  );
+
+  it('correctly removes trailing empty returns as the only function statement', () =>
+    check(`
+      -> return
+    `, `
+      (function() {  });
+    `)
+  );
 });