[dart2js] Migrate command line util

Also makes the following minor improvements:
- Standardizes the file on single quotes to make the file more uniform and match elsewhere.
- Switches the ESCAPE_MAPPING declaration to const
- Corrects method spelling in doc comment

Change-Id: Ic271cd28d287f321b9b3719250b053df548ba194
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/202061
Reviewed-by: Stephen Adams <[email protected]>
Commit-Queue: Kevin Moore <[email protected]>

pkg/compiler/lib/src/util/command_line.dart

@@ -2,34 +2,36 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.12
+
 library dart2js.util.command_line;
 
 /// The accepted escapes in the input of the --batch processor.
 ///
 /// Contrary to Dart strings it does not contain hex escapes (\u or \x).
-Map<String, String> ESCAPE_MAPPING = const {
-  "n": "\n",
-  "r": "\r",
-  "t": "\t",
-  "b": "\b",
-  "f": "\f",
-  "v": "\v",
-  "\\": "\\",
+const Map<String, String> ESCAPE_MAPPING = {
+  'n': '\n',
+  'r': '\r',
+  't': '\t',
+  'b': '\b',
+  'f': '\f',
+  'v': '\v',
+  '\\': '\\',
 };
 
-/// Splits the line similar to how a shell would split arguments. If [windows]
+/// Splits the [line] similar to how a shell would split arguments. If [windows]
 /// is `true` escapes will be handled like on the Windows command-line.
 ///
 /// Example:
 ///
-///     splitline("""--args "ab"c 'with " \'spaces'""").forEach(print);
+///     splitLine("""--args "ab"c 'with " \'spaces'""").forEach(print);
 ///     // --args
 ///     // abc
 ///     // with " 'spaces
-List<String> splitLine(String line, {bool windows: false}) {
+List<String> splitLine(String line, {bool windows = false}) {
   List<String> result = <String>[];
   bool inQuotes = false;
-  String openingQuote;
+  String? openingQuote;
   StringBuffer buffer = new StringBuffer();
   for (int i = 0; i < line.length; i++) {
     String c = line[i];
@@ -44,7 +46,7 @@ List<String> splitLine(String line, {bool windows: false}) {
     }
     if (c == '\\') {
       if (i == line.length - 1) {
-        throw new FormatException("Unfinished escape: $line");
+        throw new FormatException('Unfinished escape: $line');
       }
       if (windows) {
         String next = line[i + 1];
@@ -57,20 +59,21 @@ List<String> splitLine(String line, {bool windows: false}) {
         i++;
 
         c = line[i];
-        String mapped = ESCAPE_MAPPING[c];
-        if (mapped == null) mapped = c;
+        String mapped = ESCAPE_MAPPING[c] ?? c;
         buffer.write(mapped);
         continue;
       }
     }
-    if (!inQuotes && c == " ") {
-      if (buffer.isNotEmpty) result.add(buffer.toString());
-      buffer.clear();
+    if (!inQuotes && c == ' ') {
+      if (buffer.isNotEmpty) {
+        result.add(buffer.toString());
+        buffer.clear();
+      }
       continue;
     }
     buffer.write(c);
   }
-  if (inQuotes) throw new FormatException("Unclosed quotes: $line");
+  if (inQuotes) throw new FormatException('Unclosed quotes: $line');
   if (buffer.isNotEmpty) result.add(buffer.toString());
   return result;
 }