Implement modInverse (bigint version does not support even modulus yet).

sdk/lib/_internal/compiler/js_lib/js_number.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of _interceptors;
 
 /**
  * The super interceptor class for [JSInt] and [JSDouble]. The compiler
  * recognizes this class as an interceptor, and changes references to
  * [:this:] to actually use the receiver of the method, which is
@@ skipping 393 matching lines @@
     while (e > 0) {
       if (e.isOdd) {
         r = (r * b) % m;
       }
       e ~/= 2;
       b = (b * b) % m;
     }
     return r;
   }
 
+  // Returns 1/this % m, with m > 0.
+  int modInverse(int m) {
+    if (m is! int) throw new ArgumentError(m);
+    if (m <= 0) throw new RangeError(m);
+    if (m is _Bigint) {
+      return _toBigint().modInverse(m);
+    }
+    bool ac = m.isEven;
+    int u = m;
+    int v = this;
+    int a = 1,
+        b = 0,
+        c = 0,
+        d = 1;
+    while (u != 0) {
+      while (u.isEven) {
+        u ~/= 2;
+        if (ac) {
+          if (!a.isEven || !b.isEven) {
+            a += this;
+            b -= m;
+          }
+          a ~/= 2;
+        } else if (!b.isEven) {
+          b -= m;
+        }
+        b ~/= 2;
+      }
+      while (v.isEven) {
+        v ~/= 2;
+        if (ac) {
+          if (!c.isEven || !d.isEven) {
+            c += this;
+            d -= m;
+          }
+          c ~/= 2;
+        } else if (!d.isEven) {
+          d -= m;
+        }
+        d ~/= 2;
+      }
+      if (u >= v) {
+        u -= v;
+        if (ac) a -= c;
+        b -= d;
+      } else {
+        v -= u;
+        if (ac) c -= a;
+        d -= b;
+      }
+    }
+    if (v != 1) return 0;
+    if (d > m) return d - m;
+    if (d < 0) d += m;
+    if (d < 0) d += m;
+    return d;
+  }
+
   // Assumes i is <= 32-bit and unsigned.
   static int _bitCount(int i) {
     // See "Hacker's Delight", section 5-1, "Counting 1-Bits".
 
     // The basic strategy is to use "divide and conquer" to
     // add pairs (then quads, etc.) of bits together to obtain
     // sub-counts.
     //
     // A straightforward approach would look like:
     //
@@ skipping 34 matching lines @@
 }
 
 class JSDouble extends JSNumber implements double {
   const JSDouble();
   Type get runtimeType => double;
 }
 
 class JSPositiveInt extends JSInt {}
 class JSUInt32 extends JSPositiveInt {}
 class JSUInt31 extends JSUInt32 {}