Adds support for all attribute selectors

Fixes #237.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=157251667

lib/src/compiler/attribute_matcher.dart

@@ -0,0 +1,114 @@
+/// Matches HTML attribute values.
+///
+/// Note that matching the attribute name is left to the caller. This improves
+/// the efficiency of matching an attribute against a collection of
+/// [AttributeMatcher]s that share a name.
+///
+/// https://www.w3.org/TR/selectors4/#attribute-selectors
+abstract class AttributeMatcher {
+  final String name;
+  final String value;
+
+  const AttributeMatcher(this.name, this.value);
+
+  bool matches(String value);
+}
+
+/// Matches a value that is exactly [value].
+///
+/// https://www.w3.org/TR/selectors4/#attribute-representation
+class ExactAttributeMatcher extends AttributeMatcher {
+  ExactAttributeMatcher(String name, String value) : super(name, value);
+
+  @override
+  bool matches(String value) => value == this.value;
+
+  @override
+  String toString() => '[$name="$value"]';
+}
+
+/// Matches a value that is exactly [value] or prefixed by [value]-.
+///
+/// https://www.w3.org/TR/selectors4/#attribute-representation
+class HyphenAttributeMatcher extends AttributeMatcher {
+  final String _prefix;
+
+  HyphenAttributeMatcher(String name, String value)
+      : _prefix = '$value-',
+        super(name, value);
+
+  @override
+  bool matches(String value) =>
+      value == this.value || value.startsWith(_prefix);
+
+  @override
+  String toString() => '[$name|="$value"]';
+}
+
+/// Matches a whitespace-delimited list of words containing [value].
+///
+/// https://www.w3.org/TR/selectors4/#attribute-representation
+class ListAttributeMatcher extends AttributeMatcher {
+  static final _whitespaceRe = new RegExp(r'\s+');
+
+  ListAttributeMatcher(String name, String item) : super(name, item);
+
+  @override
+  bool matches(String value) => value.split(_whitespaceRe).contains(this.value);
+
+  @override
+  String toString() => '[$name~="$value"]';
+}
+
+/// Matches a value that begins with the prefix [value].
+///
+/// https://www.w3.org/TR/selectors4/#attribute-substrings
+class PrefixAttributeMatcher extends AttributeMatcher {
+  PrefixAttributeMatcher(String name, String prefix) : super(name, prefix);
+
+  @override
+  bool matches(String value) => value.startsWith(this.value);
+
+  @override
+  String toString() => '[$name^="$value"]';
+}
+
+/// Matches any value.
+///
+/// https://www.w3.org/TR/selectors4/#attribute-representation
+class SetAttributeMatcher extends AttributeMatcher {
+  SetAttributeMatcher(String name) : super(name, null);
+
+  @override
+  bool matches(String value) => true;
+
+  @override
+  String toString() => '[$name]';
+}
+
+/// Matches a value that contains the substring [value].
+///
+/// https://www.w3.org/TR/selectors4/#attribute-substrings
+class SubstringAttributeMatcher extends AttributeMatcher {
+  SubstringAttributeMatcher(String name, String substring)
+      : super(name, substring);
+
+  @override
+  bool matches(String value) => value.contains(this.value);
+
+  @override
+  String toString() => '[$name*="$value"]';
+}
+
+/// Matches a value that ends with the suffix [value].
+///
+/// https://www.w3.org/TR/selectors4/#attribute-substrings
+class SuffixAttributeMatcher extends AttributeMatcher {
+  SuffixAttributeMatcher(String name, String suffix) : super(name, suffix);
+
+  @override
+  bool matches(String value) => value.endsWith(this.value);
+
+  @override
+  String toString() => '[$name\$="$value"]';
+}