Create pkg/front_end/src/base and begin populating with general use classes.

pkg/front_end/lib/src/base/errors.dart

Index: pkg/front_end/lib/src/base/errors.dart
diff --git a/pkg/front_end/lib/src/scanner/errors.dart b/pkg/front_end/lib/src/base/errors.dart
similarity index 81%
copy from pkg/front_end/lib/src/scanner/errors.dart
copy to pkg/front_end/lib/src/base/errors.dart
index e5f66def1546d0a4d031edf0d72301342d6f9e05..d39953f50ab0739f94f9af899f302bc8c914e4ff 100644
--- a/pkg/front_end/lib/src/scanner/errors.dart
+++ b/pkg/front_end/lib/src/base/errors.dart
@@ -280,57 +280,3 @@ class ErrorType implements Comparable<ErrorType> {
   @override
   String toString() => name;
 }
-
-/**
- * The error codes used for errors detected by the scanner.
- */
-class ScannerErrorCode extends ErrorCode {
-  /**
-   * Parameters:
-   * 0: the illegal character
-   */
-  static const ScannerErrorCode ILLEGAL_CHARACTER =
-      const ScannerErrorCode('ILLEGAL_CHARACTER', "Illegal character '{0}'.");
-
-  static const ScannerErrorCode MISSING_DIGIT =
-      const ScannerErrorCode('MISSING_DIGIT', "Decimal digit expected.");
-
-  static const ScannerErrorCode MISSING_HEX_DIGIT = const ScannerErrorCode(
-      'MISSING_HEX_DIGIT', "Hexidecimal digit expected.");
-
-  static const ScannerErrorCode MISSING_QUOTE =
-      const ScannerErrorCode('MISSING_QUOTE', "Expected quote (' or \").");
-
-  /**
-   * Parameters:
-   * 0: the path of the file that cannot be read
-   */
-  static const ScannerErrorCode UNABLE_GET_CONTENT = const ScannerErrorCode(
-      'UNABLE_GET_CONTENT', "Unable to get content of '{0}'.");
-
-  static const ScannerErrorCode UNTERMINATED_MULTI_LINE_COMMENT =
-      const ScannerErrorCode(
-          'UNTERMINATED_MULTI_LINE_COMMENT',
-          "Unterminated multi-line comment.",
-          "Try terminating the comment with '*/', or "
-          "removing any unbalanced occurances of '/*' (because comments nest in Dart).");
-
-  static const ScannerErrorCode UNTERMINATED_STRING_LITERAL =
-      const ScannerErrorCode(
-          'UNTERMINATED_STRING_LITERAL', "Unterminated string literal.");
-
-  /**
-   * Initialize a newly created error code to have the given [name]. The message
-   * associated with the error will be created from the given [message]
-   * template. The correction associated with the error will be created from the
-   * given [correction] template.
-   */
-  const ScannerErrorCode(String name, String message, [String correction])
-      : super(name, message, correction);
-
-  @override
-  ErrorSeverity get errorSeverity => ErrorSeverity.ERROR;
-
-  @override
-  ErrorType get type => ErrorType.SYNTACTIC_ERROR;
-}