Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

pkg/webdriver/base64decoder.dart

-/**
- * A simple base64 decoder class, used to decode web browser screenshots
- * returned by WebDriver.
- */
-class Base64Decoder {
-
-  static int getVal(String s, pos) {
-    int code = s.charCodeAt(pos);
-    if (code >= 65 && code < (65+26)) { // 'A'..'Z'
-      return code - 65;
-    } else if (code >= 97 && code < (97+26)) { // 'a'..'z'
-      return code - 97 + 26;
-    } else if (code >= 48 && code < (48+10)) { // '0'..'9'
-      return code - 48 + 52;
-    } else if (code == 43) { // '+'
-      return 62;
-    } else if (code == 47) { // '/'
-      return 63;
-    } else {
-      throw 'Invalid character $s';
-    }
-  }
-
-  static List<int> decode(String s) {
-    var rtn = new List<int>();
-    var pos = 0;
-    while (pos < s.length) {
-      if (s[pos+2] =='=') { // Single byte as two chars.
-        int v = (getVal(s, pos) << 18 ) | (getVal(s, pos+1) << 12 );
-        rtn.add((v >> 16) & 0xff);
-        break;
-      } else if (s[pos+3] == '=') { // Two bytes as 3 chars.
-        int v = (getVal(s, pos) << 18 ) | (getVal(s, pos+1) << 12 ) |
-            (getVal(s, pos + 2) << 6);
-        rtn.add((v >> 16) & 0xff);
-        rtn.add((v >> 8) & 0xff);
-        break;
-      } else { // Three bytes as 4 chars.
-        int v = (getVal(s, pos) << 18 ) | (getVal(s, pos+1) << 12 ) |
-            (getVal(s, pos + 2) << 6) | getVal(s, pos+3);
-        pos += 4;
-        rtn.add((v >> 16 ) & 0xff);
-        rtn.add((v >> 8) & 0xff);
-        rtn.add(v & 0xff);
-      }
-    }
-    return rtn;
-  }
-}