Implement string literal trim() folding

Closes #2900

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194607689

Author: puddly

src/com/google/javascript/jscomp/PeepholeReplaceKnownMethods.java

@@ -218,6 +218,8 @@ private Node tryFoldKnownStringMethods(Node subtree, Node callTarget) {
             return tryFoldStringToLowerCase(subtree, stringNode);
           case "toUpperCase":
             return tryFoldStringToUpperCase(subtree, stringNode);
+          case "trim":
+            return tryFoldStringTrim(subtree, stringNode);
           default: // fall out
         }
       } else {
@@ -319,6 +321,20 @@ private Node tryFoldStringToUpperCase(Node subtree, Node stringNode) {
     return replacement;
   }
 
+  /** @return The trimmed string Node. */
+  private Node tryFoldStringTrim(Node subtree, Node stringNode) {
+    // See ECMA 15.5.4.20, 7.2, and 7.3
+    // All Unicode 10.0 whitespace + BOM
+    String whitespace =
+        "[ \t\n-\r\\u0085\\u00A0\\u1680\\u2000-\\u200A\\u2028\\u2029\\u202F\\u205F\\u3000\\uFEFF]+";
+    String trimmed =
+        stringNode.getString().replaceAll("^" + whitespace + "|" + whitespace + "$", "");
+    Node replacement = IR.string(trimmed);
+    subtree.replaceWith(replacement);
+    compiler.reportChangeToEnclosingScope(replacement);
+    return replacement;
+  }
+
   /**
    * @param input string representation of a number
    * @return string with leading and trailing zeros removed

test/com/google/javascript/jscomp/PeepholeIntegrationTest.java

@@ -377,6 +377,7 @@ public void testTemplateStringsKnownMethods() {
     test("x = `abcdef`.charCodeAt(0)", "x = 97");
     test("x = `abc`.toUpperCase()", "x = 'ABC'");
     test("x = `ABC`.toLowerCase()", "x = 'abc'");
+    test("x = `\t\n\uFEFF\t asd foo bar \r\n`.trim()", "x = 'asd foo bar'");
     test("x = parseInt(`123`)", "x = 123");
     test("x = parseFloat(`1.23`)", "x = 1.23");
   }