Provide cross-browser Rects for box model dimensions for Elements.

tools/dom/src/Dimension.dart

+/**
+ * Class representing a
+ * [length measurement](https://developer.mozilla.org/en-US/docs/Web/CSS/length)
+ * in CSS.
+ */
+@Experimental()
+class Dimension {
+  num _value;
+  String _unit;
+
+  /** Set this CSS Dimension to a percentage `value`. */
+  Dimension.percent(this._value) : _unit = '%';
+
+  /** Set this CSS Dimension to a pixel `value`. */
+  Dimension.px(this._value) : _unit = 'px';
+
+  /** Set this CSS Dimension to a pica `value`. */
+  Dimension.pc(this._value) : _unit = 'pc';
+
+  /** Set this CSS Dimension to a point `value`. */
+  Dimension.pt(this._value) : _unit = 'pt';
+
+  /** Set this CSS Dimension to an inch `value`. */
+  Dimension.inch(this._value) : _unit = 'in';
+
+  /** Set this CSS Dimension to a centimeter `value`. */
+  Dimension.cm(this._value) : _unit = 'cm';
+
+  /** Set this CSS Dimension to a millimeter `value`. */
+  Dimension.mm(this._value) : _unit = 'mm';
+
+  /**
+   * Set this CSS Dimension to the specified number of ems.
+   *
+   * 1em is equal to the current font size. (So 2ems is equal to double the font
+   * size). This is useful for producing website layouts that scale nicely with
+   * the user's desired font size.
+   */
+  Dimension.em(this._value) : _unit = 'em';
+
+  /**
+   * Set this CSS Dimension to the specified number of x-heights.
+   *
+   * One ex is equal to the the x-height of a font's baseline to its mean line,
+   * generally the height of the letter "x" in the font, which is usually about
+   * half the font-size.
+   */
+  Dimension.ex(this._value) : _unit = 'ex';
+
+  /**
+   * Construct a Dimension object from the valid, simple CSS string `cssValue`
+   * that represents a distance measurement.
+   *
+   * This constructor is intended as a convenience method for working with
+   * simplistic CSS length measurements. Non-numeric values such as `auto` or
+   * `inherit` or invalid CSS will cause this constructor to throw a
+   * FormatError.
+   */
+  Dimension.css(String cssValue) {
+    if (cssValue == '') cssValue = '0px';
+    if (cssValue.endsWith('%')) {
+      _unit = '%';
+    } else {
+      _unit = cssValue.substring(cssValue.length - 2);
+    }
+    if (cssValue.contains('.')) {
+      _value = double.parse(cssValue.substring(0,
+          cssValue.length - _unit.length));
+    } else {
+      _value = int.parse(cssValue.substring(0, cssValue.length - _unit.length));
+    }
+  }
+
+  /** Print out the CSS String representation of this value. */
+  String toString() {
+    return '${_value}${_unit}';
+  }
+
+  /** Return a unitless, numerical value of this CSS value. */
+  num get value => this._value;
+}