feat(StringWrapper): add support for JS slice method to string

modules/angular2/src/core/facade/lang.dart

@@ -94,6 +94,14 @@ class StringWrapper {
     return s.startsWith(start);
   }
 
+  static String slice(String s, [int start = 0, int end]) {
+    //in JS if start > end an empty string is returned
+    if(end != null && start > end) {
+      return "";
+    }
+    return s.substring(_startOffset(s, start), _endOffset(s, end));
+  }
+
   static String substring(String s, int start, [int end]) {
     return s.substring(start, end);
   }
@@ -107,6 +115,21 @@ class StringWrapper {
   }
 
   static int compare(String a, String b) => a.compareTo(b);
+
+  // JS slice function can take start < 0 which indicates a position relative to
+  // the end of the string
+  static int _startOffset(String s, int start) {
+    int len = s.length;
+    return start = start < 0 ? math.max(len + start, 0) : math.min(start, len);
+  }
+
+  // JS slice function can take end < 0 which indicates a position relative to
+  // the end of the string
+  static int _endOffset(String s, int end) {
+    int len = s.length;
+    if (end == null) return len;
+    return end < 0 ? math.max(len + end, 0) : math.min(end, len);
+  }
 }
 
 class StringJoiner {

modules/angular2/src/core/facade/lang.ts

@@ -151,6 +151,10 @@ export class StringWrapper {
     return s.replace(from, replace);
   }
 
+  static slice<T>(s: string, from: number = 0, to: number = null): string {
+    return s.slice(from, to === null ? undefined : to);
+  }
+
   static toUpperCase(s: string): string { return s.toUpperCase(); }
 
   static toLowerCase(s: string): string { return s.toLowerCase(); }

modules/angular2/test/core/facade/lang_spec.ts

@@ -42,7 +42,7 @@ export function main() {
   });
 
   describe('String', () => {
-    var upper, lower;
+    var upper, lower, s;
 
     beforeEach(() => {
       upper = 'SOMETHING';
@@ -60,5 +60,25 @@ export function main() {
 
       expect(str).toEqual(lower);
     });
+
+    describe('slice', () => {
+      beforeEach(() => { s = "abcdefghij"; });
+
+      it('should return the whole string if neither start nor end are specified',
+         () => { expect(StringWrapper.slice(s)).toEqual("abcdefghij"); });
+
+      it('should return up to the end if end is not specified',
+         () => { expect(StringWrapper.slice(s, 1)).toEqual("bcdefghij"); });
+
+      it('should support negative start',
+         () => { expect(StringWrapper.slice(s, -1)).toEqual("j"); });
+
+      it('should support negative end',
+         () => { expect(StringWrapper.slice(s, -3, -1)).toEqual("hi"); });
+
+      it('should return empty string if start is greater than end',
+         () => { expect(StringWrapper.slice(s, 4, 2)).toEqual(""); });
+    });
+
   });
 }